diff --git a/build/NullableEnable.props b/build/NullableEnable.props
new file mode 100644
index 0000000000..cf5f9069d4
--- /dev/null
+++ b/build/NullableEnable.props
@@ -0,0 +1,11 @@
+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <!-- 
+    The Nullable annotations on netstandard2.0 are incomplete and incorrect in places. Ignore
+    nullable warnings on netstandard2.0 and make them errors on later target frameworks.
+  -->
+  <PropertyGroup>
+    <Nullable>enable</Nullable>
+    <WarningsAsErrors Condition="'$(TargetFramework)' != 'netstandard2.0'">$(WarningsAsErrors);nullable</WarningsAsErrors>
+    <NoWarn Condition="'$(TargetFramework)' == 'netstandard2.0'">$(NoWarn);nullable</NoWarn>
+  </PropertyGroup>
+</Project>
diff --git a/src/Avalonia.Diagnostics/Avalonia.Diagnostics.csproj b/src/Avalonia.Diagnostics/Avalonia.Diagnostics.csproj
index 35de491668..b7452af111 100644
--- a/src/Avalonia.Diagnostics/Avalonia.Diagnostics.csproj
+++ b/src/Avalonia.Diagnostics/Avalonia.Diagnostics.csproj
@@ -3,7 +3,6 @@
     <TargetFramework>netstandard2.0</TargetFramework>
     <RootNamespace>Avalonia</RootNamespace>
     <PackageId>Avalonia.Diagnostics</PackageId>
-    <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Update="**\*.xaml.cs">
@@ -34,4 +33,5 @@
   <Import Project="..\..\build\Rx.props" />
   <Import Project="..\..\build\BuildTargets.targets" />
   <Import Project="..\..\build\ApiDiff.props" />
+  <Import Project="..\..\build\NullableEnable.props" />
 </Project>
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/AvaloniaPropertyViewModel.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/AvaloniaPropertyViewModel.cs
index 04fc038e2b..c23fdc166a 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/AvaloniaPropertyViewModel.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/AvaloniaPropertyViewModel.cs
@@ -34,7 +34,7 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public override System.Type Type => _type;
 
-        public override string Value
+        public override string? Value
         {
             get => ConvertToString(_value);
             set
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ClrPropertyViewModel.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ClrPropertyViewModel.cs
index d5748c1d8d..8cf1be154d 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ClrPropertyViewModel.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ClrPropertyViewModel.cs
@@ -35,7 +35,7 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public override System.Type Type => _type;
 
-        public override string Value 
+        public override string? Value 
         {
             get => ConvertToString(_value);
             set
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlDetailsViewModel.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlDetailsViewModel.cs
index f18967356d..acb51e9e27 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlDetailsViewModel.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlDetailsViewModel.cs
@@ -437,7 +437,7 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
         
-        protected  void NavigateToProperty(object o, string entityName)
+        protected  void NavigateToProperty(object o, string? entityName)
         {
             var oldSelectedEntity = SelectedEntity;
             if (oldSelectedEntity is IAvaloniaObject ao1)
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/PropertyViewModel.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/PropertyViewModel.cs
index 557ad64d86..d5909d0553 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/PropertyViewModel.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/PropertyViewModel.cs
@@ -16,12 +16,12 @@ namespace Avalonia.Diagnostics.ViewModels
         public abstract string Group { get; }
         public abstract Type Type { get; }
         public abstract Type? DeclaringType { get; }
-        public abstract string Value { get; set; }
+        public abstract string? Value { get; set; }
         public abstract string Priority { get; }
         public abstract bool? IsAttached { get;  }
         public abstract void Update();        
 
-        protected static string ConvertToString(object? value)
+        protected static string? ConvertToString(object? value)
         {
             if (value is null)
             {
@@ -59,8 +59,13 @@ namespace Avalonia.Diagnostics.ViewModels
             throw new InvalidCastException("Unable to convert value.");
         }
 
-        protected static object? ConvertFromString(string s, Type targetType)
+        protected static object? ConvertFromString(string? s, Type targetType)
         {
+            if (s is null)
+            {
+                return null;
+            }
+
             var converter = TypeDescriptor.GetConverter(targetType);
 
             if (converter.CanConvertFrom(typeof(string)))
diff --git a/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs b/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs
index ea06c33e4d..62bfdb5186 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs
@@ -17,7 +17,7 @@ namespace Avalonia.Diagnostics.Views
 {
     internal class MainWindow : Window, IStyleHost
     {
-        private readonly IDisposable _keySubscription;
+        private readonly IDisposable? _keySubscription;
         private readonly Dictionary<Popup, IDisposable> _frozenPopupStates;
         private TopLevel? _root;
 
@@ -25,7 +25,7 @@ namespace Avalonia.Diagnostics.Views
         {
             InitializeComponent();
 
-            _keySubscription = InputManager.Instance.Process
+            _keySubscription = InputManager.Instance?.Process
                 .OfType<RawKeyEventArgs>()
                 .Where(x => x.Type == RawKeyEventType.KeyDown)
                 .Subscribe(RawKeyDown);
@@ -82,7 +82,7 @@ namespace Avalonia.Diagnostics.Views
         protected override void OnClosed(EventArgs e)
         {
             base.OnClosed(e);
-            _keySubscription.Dispose();
+            _keySubscription?.Dispose();
 
             foreach (var state in _frozenPopupStates)
             {
diff --git a/src/Avalonia.Input/AccessKeyHandler.cs b/src/Avalonia.Input/AccessKeyHandler.cs
index 5082265ea6..24feb357b9 100644
--- a/src/Avalonia.Input/AccessKeyHandler.cs
+++ b/src/Avalonia.Input/AccessKeyHandler.cs
@@ -133,7 +133,7 @@ namespace Avalonia.Input
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        protected virtual void OnPreviewKeyDown(object sender, KeyEventArgs e)
+        protected virtual void OnPreviewKeyDown(object? sender, KeyEventArgs e)
         {
             if (e.Key == Key.LeftAlt || e.Key == Key.RightAlt)
             {
@@ -172,7 +172,7 @@ namespace Avalonia.Input
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        protected virtual void OnKeyDown(object sender, KeyEventArgs e)
+        protected virtual void OnKeyDown(object? sender, KeyEventArgs e)
         {
             bool menuIsOpen = MainMenu?.IsOpen == true;
 
@@ -207,7 +207,7 @@ namespace Avalonia.Input
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        protected virtual void OnPreviewKeyUp(object sender, KeyEventArgs e)
+        protected virtual void OnPreviewKeyUp(object? sender, KeyEventArgs e)
         {
             switch (e.Key)
             {
@@ -234,7 +234,7 @@ namespace Avalonia.Input
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        protected virtual void OnPreviewPointerPressed(object sender, PointerEventArgs e)
+        protected virtual void OnPreviewPointerPressed(object? sender, PointerEventArgs e)
         {
             if (_showingAccessKeys)
             {
@@ -251,7 +251,7 @@ namespace Avalonia.Input
             _owner!.ShowAccessKeys = _showingAccessKeys = false;
         }
 
-        private void MainMenuClosed(object sender, EventArgs e)
+        private void MainMenuClosed(object? sender, EventArgs e)
         {
             _owner!.ShowAccessKeys = false;
         }
diff --git a/src/Avalonia.Input/Avalonia.Input.csproj b/src/Avalonia.Input/Avalonia.Input.csproj
index cc487e285b..cf7b0b91ab 100644
--- a/src/Avalonia.Input/Avalonia.Input.csproj
+++ b/src/Avalonia.Input/Avalonia.Input.csproj
@@ -2,6 +2,7 @@
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
     <Nullable>Enable</Nullable>
+    <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="..\Avalonia.Base\Metadata\NullableAttributes.cs" Link="NullableAttributes.cs" />
@@ -15,4 +16,5 @@
   </ItemGroup>  
   <Import Project="..\..\build\Rx.props" />
   <Import Project="..\..\build\ApiDiff.props" />
+  <Import Project="..\..\build\NullableEnable.props" />
 </Project>
diff --git a/src/Avalonia.Input/FocusManager.cs b/src/Avalonia.Input/FocusManager.cs
index 2efd33df1d..177e6035b0 100644
--- a/src/Avalonia.Input/FocusManager.cs
+++ b/src/Avalonia.Input/FocusManager.cs
@@ -215,8 +215,11 @@ namespace Avalonia.Input
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private static void OnPreviewPointerPressed(object sender, RoutedEventArgs e)
+        private static void OnPreviewPointerPressed(object? sender, RoutedEventArgs e)
         {
+            if (sender is null)
+                return;
+
             var ev = (PointerPressedEventArgs)e;
             var visual = (IVisual)sender;
 
diff --git a/src/Avalonia.Input/KeyboardNavigationHandler.cs b/src/Avalonia.Input/KeyboardNavigationHandler.cs
index 6493777105..b2ebee9ef5 100644
--- a/src/Avalonia.Input/KeyboardNavigationHandler.cs
+++ b/src/Avalonia.Input/KeyboardNavigationHandler.cs
@@ -98,7 +98,7 @@ namespace Avalonia.Input
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        protected virtual void OnKeyDown(object sender, KeyEventArgs e)
+        protected virtual void OnKeyDown(object? sender, KeyEventArgs e)
         {
             var current = FocusManager.Instance.Current;
 
diff --git a/src/Avalonia.Input/MouseDevice.cs b/src/Avalonia.Input/MouseDevice.cs
index d798193866..1ed861887f 100644
--- a/src/Avalonia.Input/MouseDevice.cs
+++ b/src/Avalonia.Input/MouseDevice.cs
@@ -298,10 +298,10 @@ namespace Avalonia.Input
             root = root ?? throw new ArgumentNullException(nameof(root));
 
             var hit = HitTest(root, p);
+            var source = GetSource(hit);
 
-            if (hit != null)
+            if (source is not null)
             {
-                var source = GetSource(hit);
                 var e = new PointerReleasedEventArgs(source, _pointer, root, p, timestamp, props, inputModifiers,
                     _lastMouseDownButton);
 
@@ -321,10 +321,10 @@ namespace Avalonia.Input
             root = root ?? throw new ArgumentNullException(nameof(root));
 
             var hit = HitTest(root, p);
+            var source = GetSource(hit);
 
-            if (hit != null)
+            if (source is not null)
             {
-                var source = GetSource(hit);
                 var e = new PointerWheelEventArgs(source, _pointer, root, p, timestamp, props, inputModifiers, delta);
 
                 source?.RaiseEvent(e);
@@ -334,9 +334,10 @@ namespace Avalonia.Input
             return false;
         }
 
-        private IInteractive GetSource(IVisual hit)
+        private IInteractive? GetSource(IVisual? hit)
         {
-            hit = hit ?? throw new ArgumentNullException(nameof(hit));
+            if (hit is null)
+                return null;
 
             return _pointer.Captured ??
                 (hit as IInteractive) ??
diff --git a/src/Avalonia.Input/Pointer.cs b/src/Avalonia.Input/Pointer.cs
index a477711584..433b275ce4 100644
--- a/src/Avalonia.Input/Pointer.cs
+++ b/src/Avalonia.Input/Pointer.cs
@@ -59,7 +59,7 @@ namespace Avalonia.Input
             return parent as IInputElement ?? parent.FindAncestorOfType<IInputElement>();
         }
 
-        private void OnCaptureDetached(object sender, VisualTreeAttachmentEventArgs e)
+        private void OnCaptureDetached(object? sender, VisualTreeAttachmentEventArgs e)
         {
             Capture(GetNextCapture(e.Parent));
         }
diff --git a/src/Avalonia.Input/TextInput/InputMethodManager.cs b/src/Avalonia.Input/TextInput/InputMethodManager.cs
index dafd397348..64422a7fdf 100644
--- a/src/Avalonia.Input/TextInput/InputMethodManager.cs
+++ b/src/Avalonia.Input/TextInput/InputMethodManager.cs
@@ -8,7 +8,6 @@ namespace Avalonia.Input.TextInput
         private ITextInputMethodImpl? _im;
         private IInputElement? _focusedElement;
         private ITextInputMethodClient? _client;
-        private IDisposable? _subscribeDisposable;
         private readonly TransformTrackingHelper _transformTracker = new TransformTrackingHelper();
 
         public TextInputMethodManager()
@@ -64,7 +63,7 @@ namespace Avalonia.Input.TextInput
             }
         }
 
-        private void OnTextViewVisualChanged(object sender, EventArgs e) 
+        private void OnTextViewVisualChanged(object? sender, EventArgs e) 
             => _transformTracker.SetVisual(_client?.TextViewVisual);
 
         private void UpdateCursorRect()
@@ -79,7 +78,7 @@ namespace Avalonia.Input.TextInput
                 _im.SetCursorRect(_client.CursorRectangle.TransformToAABB(transform.Value));
         }
 
-        private void OnCursorRectangleChanged(object sender, EventArgs e)
+        private void OnCursorRectangleChanged(object? sender, EventArgs e)
         {
             if (sender == _client)
                 UpdateCursorRect();
diff --git a/src/Avalonia.Input/TextInput/TransformTrackingHelper.cs b/src/Avalonia.Input/TextInput/TransformTrackingHelper.cs
index 4211360a8f..325c72bcb5 100644
--- a/src/Avalonia.Input/TextInput/TransformTrackingHelper.cs
+++ b/src/Avalonia.Input/TextInput/TransformTrackingHelper.cs
@@ -80,7 +80,7 @@ namespace Avalonia.Input.TextInput
             }
         }
 
-        private void OnAttachedToVisualTree(object sender, VisualTreeAttachmentEventArgs visualTreeAttachmentEventArgs)
+        private void OnAttachedToVisualTree(object? sender, VisualTreeAttachmentEventArgs visualTreeAttachmentEventArgs)
         {
             SubscribeToParents();
             UpdateMatrix();
@@ -94,13 +94,13 @@ namespace Avalonia.Input.TextInput
             Dispatcher.UIThread.Post(UpdateMatrix, DispatcherPriority.Render);
         }
 
-        private void PropertyChangedHandler(object sender, AvaloniaPropertyChangedEventArgs e)
+        private void PropertyChangedHandler(object? sender, AvaloniaPropertyChangedEventArgs e)
         {
             if (e.IsEffectiveValueChange && e.Property == Visual.BoundsProperty)
                 EnqueueForUpdate();
         }
 
-        private void OnDetachedFromVisualTree(object sender, VisualTreeAttachmentEventArgs visualTreeAttachmentEventArgs)
+        private void OnDetachedFromVisualTree(object? sender, VisualTreeAttachmentEventArgs visualTreeAttachmentEventArgs)
         {
             UnsubscribeFromParents();
             UpdateMatrix();
diff --git a/src/Avalonia.Interactivity/Avalonia.Interactivity.csproj b/src/Avalonia.Interactivity/Avalonia.Interactivity.csproj
index 2da23410d6..0d4e9235c3 100644
--- a/src/Avalonia.Interactivity/Avalonia.Interactivity.csproj
+++ b/src/Avalonia.Interactivity/Avalonia.Interactivity.csproj
@@ -3,6 +3,7 @@
     <TargetFramework>netstandard2.0</TargetFramework>
     <Nullable>Enable</Nullable>
     <WarningsAsErrors>CS8600;CS8602;CS8603</WarningsAsErrors>
+    <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
   </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\Avalonia.Animation\Avalonia.Animation.csproj" />
@@ -12,4 +13,5 @@
   </ItemGroup>
   <Import Project="..\..\build\Rx.props" />
   <Import Project="..\..\build\ApiDiff.props" />
+  <Import Project="..\..\build\NullableEnable.props" />
 </Project>
\ No newline at end of file
diff --git a/src/Avalonia.Interactivity/RoutedEvent.cs b/src/Avalonia.Interactivity/RoutedEvent.cs
index 3f38a61048..a9b7dc8c89 100644
--- a/src/Avalonia.Interactivity/RoutedEvent.cs
+++ b/src/Avalonia.Interactivity/RoutedEvent.cs
@@ -120,7 +120,7 @@ namespace Avalonia.Interactivity
             bool handledEventsToo = false)
             where TTarget : class, IInteractive
         {
-            void Adapter(object sender, RoutedEventArgs e)
+            void Adapter(object? sender, RoutedEventArgs e)
             {
                 if (sender is TTarget target && e is TEventArgs args)
                 {
@@ -136,7 +136,7 @@ namespace Avalonia.Interactivity
             RoutingStrategies routes = RoutingStrategies.Direct | RoutingStrategies.Bubble,
             bool handledEventsToo = false) where TTarget : class, IInteractive
         {
-            void Adapter(object sender, RoutedEventArgs e)
+            void Adapter(object? sender, RoutedEventArgs e)
             {
                 if (sender is TTarget target && e is TEventArgs args)
                 {
diff --git a/src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj b/src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj
index 2792ae1c91..90aa6f6f4e 100644
--- a/src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj
+++ b/src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj
@@ -3,8 +3,6 @@
     <TargetFramework>netstandard2.0</TargetFramework>
     <PackageId>Avalonia.ReactiveUI</PackageId>
     <SignAssembly>false</SignAssembly>
-    <Nullable>enable</Nullable>
-    <WarningsAsErrors>nullable</WarningsAsErrors>
   </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\packages\Avalonia\Avalonia.csproj" />
@@ -12,4 +10,5 @@
   <Import Project="..\..\build\Rx.props" />
   <Import Project="..\..\build\ReactiveUI.props" />
   <Import Project="..\..\build\ApiDiff.props" />
+  <Import Project="..\..\build\NullableEnable.props" />
 </Project>
diff --git a/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj b/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
index 7b9cd0212e..f1661f53af 100644
--- a/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
+++ b/src/Markup/Avalonia.Markup/Avalonia.Markup.csproj
@@ -2,8 +2,6 @@
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
     <RootNamespace>Avalonia</RootNamespace>
-    <Nullable>Enable</Nullable>
-    <WarningsAsErrors>CS8600;CS8602;CS8603</WarningsAsErrors>
   </PropertyGroup>
   <ItemGroup>
     <None Remove="Markup\Parsers\Nodes\ExpressionGrammer" />
@@ -19,4 +17,5 @@
   <Import Project="..\..\..\build\Rx.props" />
   <Import Project="..\..\..\build\System.Memory.props" />
   <Import Project="..\..\..\build\ApiDiff.props" />
+  <Import Project="..\..\..\build\NullableEnable.props" />
 </Project>
diff --git a/src/Markup/Avalonia.Markup/Data/BindingBase.cs b/src/Markup/Avalonia.Markup/Data/BindingBase.cs
index e69d9570e2..0cffde6043 100644
--- a/src/Markup/Avalonia.Markup/Data/BindingBase.cs
+++ b/src/Markup/Avalonia.Markup/Data/BindingBase.cs
@@ -272,7 +272,7 @@ namespace Avalonia.Data
                 _target.PropertyChanged -= PropertyChanged;
             }
 
-            private void PropertyChanged(object sender, AvaloniaPropertyChangedEventArgs e)
+            private void PropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
             {
                 if (e.Property == _property)
                 {
diff --git a/src/Markup/Avalonia.Markup/Data/TemplateBinding.cs b/src/Markup/Avalonia.Markup/Data/TemplateBinding.cs
index 8c33a7b721..265af3c5da 100644
--- a/src/Markup/Avalonia.Markup/Data/TemplateBinding.cs
+++ b/src/Markup/Avalonia.Markup/Data/TemplateBinding.cs
@@ -160,7 +160,7 @@ namespace Avalonia.Data
             PublishValue();
         }
 
-        private void TargetPropertyChanged(object sender, AvaloniaPropertyChangedEventArgs e)
+        private void TargetPropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
         {
             if (e.Property == StyledElement.TemplatedParentProperty)
             {
@@ -173,7 +173,7 @@ namespace Avalonia.Data
             }
         }
 
-        private void TemplatedParentPropertyChanged(object sender, AvaloniaPropertyChangedEventArgs e)
+        private void TemplatedParentPropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
         {
             if (e.Property == Property)
             {
diff --git a/src/Markup/Avalonia.Markup/Markup/Data/DelayedBinding.cs b/src/Markup/Avalonia.Markup/Markup/Data/DelayedBinding.cs
index 0b0ed7b06a..84fd16b743 100644
--- a/src/Markup/Avalonia.Markup/Markup/Data/DelayedBinding.cs
+++ b/src/Markup/Avalonia.Markup/Markup/Data/DelayedBinding.cs
@@ -99,9 +99,9 @@ namespace Avalonia.Markup.Data
             }
         }
 
-        private static void ApplyBindings(object sender, EventArgs e)
+        private static void ApplyBindings(object? sender, EventArgs e)
         {
-            var target = (IStyledElement)sender;
+            var target = (IStyledElement)sender!;
             ApplyBindings(target);
             target.Initialized -= ApplyBindings;
         }
diff --git a/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/StringIndexerNode.cs b/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/StringIndexerNode.cs
index 11b0df2f35..ceb0812a9a 100644
--- a/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/StringIndexerNode.cs
+++ b/src/Markup/Avalonia.Markup/Markup/Parsers/Nodes/StringIndexerNode.cs
@@ -297,7 +297,7 @@ namespace Avalonia.Markup.Parsers.Nodes
 
         protected override bool ShouldUpdate(object? sender, PropertyChangedEventArgs e)
         {
-            if (sender is null)
+            if (sender is null || e.PropertyName is null)
                 return false;
             var typeInfo = sender.GetType().GetTypeInfo();
             return typeInfo.GetDeclaredProperty(e.PropertyName)?.GetIndexParameters().Any() ?? false;
diff --git a/src/Shared/PlatformSupport/AssetLoader.cs b/src/Shared/PlatformSupport/AssetLoader.cs
index 90b341297c..affb861de0 100644
--- a/src/Shared/PlatformSupport/AssetLoader.cs
+++ b/src/Shared/PlatformSupport/AssetLoader.cs
@@ -17,7 +17,7 @@ namespace Avalonia.Shared.PlatformSupport
         private static readonly Dictionary<string, AssemblyDescriptor> AssemblyNameCache
             = new Dictionary<string, AssemblyDescriptor>();
 
-        private AssemblyDescriptor _defaultResmAssembly;
+        private AssemblyDescriptor? _defaultResmAssembly;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="AssetLoader"/> class.
@@ -25,7 +25,7 @@ namespace Avalonia.Shared.PlatformSupport
         /// <param name="assembly">
         /// The default assembly from which to load resm: assets for which no assembly is specified.
         /// </param>
-        public AssetLoader(Assembly assembly = null)
+        public AssetLoader(Assembly? assembly = null)
         {
             if (assembly == null)
                 assembly = Assembly.GetEntryAssembly();
@@ -50,7 +50,7 @@ namespace Avalonia.Shared.PlatformSupport
         /// A base URI to use if <paramref name="uri"/> is relative.
         /// </param>
         /// <returns>True if the asset could be found; otherwise false.</returns>
-        public bool Exists(Uri uri, Uri baseUri = null)
+        public bool Exists(Uri uri, Uri? baseUri = null)
         {
             return GetAsset(uri, baseUri) != null;
         }
@@ -66,7 +66,7 @@ namespace Avalonia.Shared.PlatformSupport
         /// <exception cref="FileNotFoundException">
         /// The asset could not be found.
         /// </exception>
-        public Stream Open(Uri uri, Uri baseUri = null) => OpenAndGetAssembly(uri, baseUri).Item1;
+        public Stream Open(Uri uri, Uri? baseUri = null) => OpenAndGetAssembly(uri, baseUri).Item1;
 
         /// <summary>
         /// Opens the asset with the requested URI and returns the asset stream and the
@@ -82,7 +82,7 @@ namespace Avalonia.Shared.PlatformSupport
         /// <exception cref="FileNotFoundException">
         /// The asset could not be found.
         /// </exception>
-        public (Stream stream, Assembly assembly) OpenAndGetAssembly(Uri uri, Uri baseUri = null)
+        public (Stream stream, Assembly assembly) OpenAndGetAssembly(Uri uri, Uri? baseUri = null)
         {
             var asset = GetAsset(uri, baseUri);
 
@@ -94,11 +94,11 @@ namespace Avalonia.Shared.PlatformSupport
             return (asset.GetStream(), asset.Assembly);
         }
 
-        public Assembly GetAssembly(Uri uri, Uri baseUri)
+        public Assembly? GetAssembly(Uri uri, Uri baseUri)
         {
             if (!uri.IsAbsoluteUri && baseUri != null)
                 uri = new Uri(baseUri, uri);
-            return GetAssembly(uri).Assembly;
+            return GetAssembly(uri)?.Assembly;
         }
 
         /// <summary>
@@ -113,7 +113,7 @@ namespace Avalonia.Shared.PlatformSupport
             {
                 var assembly = GetAssembly(uri);
 
-                return assembly?.Resources.Where(x => x.Key.Contains(uri.AbsolutePath))
+                return assembly?.Resources?.Where(x => x.Key.Contains(uri.AbsolutePath))
                            .Select(x =>new Uri($"resm:{x.Key}?assembly={assembly.Name}")) ??
                        Enumerable.Empty<Uri>();
             }
@@ -139,7 +139,7 @@ namespace Avalonia.Shared.PlatformSupport
             return Enumerable.Empty<Uri>();
         }
 
-        private Uri EnsureAbsolute(Uri uri, Uri baseUri)
+        private Uri EnsureAbsolute(Uri uri, Uri? baseUri)
         {
             if (uri.IsAbsoluteUri)
                 return uri;
@@ -153,7 +153,7 @@ namespace Avalonia.Shared.PlatformSupport
             return new Uri(baseUri, uri);
         }
         
-        private IAssetDescriptor GetAsset(Uri uri, Uri baseUri)
+        private IAssetDescriptor? GetAsset(Uri uri, Uri? baseUri)
         {           
             if (uri.IsAbsoluteUri && uri.Scheme == "resm")
             {
@@ -166,10 +166,9 @@ namespace Avalonia.Shared.PlatformSupport
                         "don't know where to look up for the resource, try specifying assembly explicitly.");
                 }
 
-                IAssetDescriptor rv;
-
                 var resourceKey = uri.AbsolutePath;
-                asm.Resources.TryGetValue(resourceKey, out rv);
+                IAssetDescriptor? rv = null;
+                asm.Resources?.TryGetValue(resourceKey, out rv);
                 return rv;
             }
 
@@ -193,7 +192,7 @@ namespace Avalonia.Shared.PlatformSupport
             return (asm, uri.AbsolutePath);
         }
         
-        private AssemblyDescriptor GetAssembly(Uri uri)
+        private AssemblyDescriptor? GetAssembly(Uri? uri)
         {
             if (uri != null)
             {
@@ -205,9 +204,8 @@ namespace Avalonia.Shared.PlatformSupport
                 if (uri.Scheme == "resm")
                 {
                     var qs = ParseQueryString(uri);
-                    string assemblyName;
 
-                    if (qs.TryGetValue("assembly", out assemblyName))
+                    if (qs.TryGetValue("assembly", out var assemblyName))
                     {
                         return GetAssembly(assemblyName);
                     }
@@ -222,8 +220,7 @@ namespace Avalonia.Shared.PlatformSupport
             if (name == null)
                 throw new ArgumentNullException(nameof(name));
 
-            AssemblyDescriptor rv;
-            if (!AssemblyNameCache.TryGetValue(name, out rv))
+            if (!AssemblyNameCache.TryGetValue(name, out var rv))
             {
                 var loadedAssemblies = AppDomain.CurrentDomain.GetAssemblies();
                 var match = loadedAssemblies.FirstOrDefault(a => a.GetName().Name == name);
@@ -275,7 +272,8 @@ namespace Avalonia.Shared.PlatformSupport
 
             public Stream GetStream()
             {
-                return _asm.GetManifestResourceStream(_name);
+                var s = _asm.GetManifestResourceStream(_name);
+                return s ?? throw new InvalidOperationException($"Could not find manifest resource stream '{_name}',");
             }
 
             public Assembly Assembly => _asm;
@@ -296,7 +294,9 @@ namespace Avalonia.Shared.PlatformSupport
             
             public Stream GetStream()
             {
-                return new SlicedStream(Assembly.GetManifestResourceStream(AvaloniaResourceName), _offset, _length);
+                var s = Assembly.GetManifestResourceStream(AvaloniaResourceName) ??
+                    throw new InvalidOperationException($"Could not find manifest resource stream '{AvaloniaResourceName}',");
+                return new SlicedStream(s, _offset, _length);
             }
         }
         
@@ -383,9 +383,9 @@ namespace Avalonia.Shared.PlatformSupport
             }
 
             public Assembly Assembly { get; }
-            public Dictionary<string, IAssetDescriptor> Resources { get; }
-            public Dictionary<string, IAssetDescriptor> AvaloniaResources { get; }
-            public string Name { get; }
+            public Dictionary<string, IAssetDescriptor>? Resources { get; }
+            public Dictionary<string, IAssetDescriptor>? AvaloniaResources { get; }
+            public string? Name { get; }
         }
         
         public static void RegisterResUriParsers()
diff --git a/src/Web/Avalonia.Web.Blazor/Avalonia.Web.Blazor.csproj b/src/Web/Avalonia.Web.Blazor/Avalonia.Web.Blazor.csproj
index 8b7babe5b1..94350b40cf 100644
--- a/src/Web/Avalonia.Web.Blazor/Avalonia.Web.Blazor.csproj
+++ b/src/Web/Avalonia.Web.Blazor/Avalonia.Web.Blazor.csproj
@@ -2,7 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>net6.0</TargetFramework>
-    <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <PackageId>Avalonia.Web.Blazor</PackageId>
     <LangVersion>preview</LangVersion>
@@ -32,7 +31,8 @@
   <Import Project="..\..\..\build\BuildTargets.targets" />
   <Import Project="..\..\..\build\SkiaSharp.props" />
   <Import Project="..\..\..\build\HarfBuzzSharp.props" />
-
+  <Import Project="..\..\..\build\NullableEnable.props" />
+  
   <ItemGroup>
     <Content Include="*.props">
       <Pack>true</Pack>
diff --git a/src/Web/Avalonia.Web.Blazor/WindowingPlatform.cs b/src/Web/Avalonia.Web.Blazor/WindowingPlatform.cs
index a3f2de5b61..4439ec161c 100644
--- a/src/Web/Avalonia.Web.Blazor/WindowingPlatform.cs
+++ b/src/Web/Avalonia.Web.Blazor/WindowingPlatform.cs
@@ -12,7 +12,7 @@ namespace Avalonia.Web.Blazor
     public class BlazorWindowingPlatform : IWindowingPlatform, IPlatformSettings, IPlatformThreadingInterface
     {
         private bool _signaled;
-        private static int s_uiThreadId = -1;
+        private static KeyboardDevice? s_keyboard;
 
         public IWindowImpl CreateWindow() => throw new NotSupportedException();
 
@@ -26,16 +26,17 @@ namespace Avalonia.Web.Blazor
             return null;
         }
 
-        public static KeyboardDevice Keyboard { get; private set; }
+        public static KeyboardDevice Keyboard => s_keyboard ??
+            throw new InvalidOperationException("BlazorWindowingPlatform not registered.");
 
         public static void Register()
         {
             var instance = new BlazorWindowingPlatform();
-            Keyboard = new KeyboardDevice();
+            s_keyboard = new KeyboardDevice();
             AvaloniaLocator.CurrentMutable
                 .Bind<IClipboard>().ToSingleton<ClipboardStub>()
                 .Bind<ICursorFactory>().ToSingleton<CursorFactoryStub>()
-                .Bind<IKeyboardDevice>().ToConstant(Keyboard)
+                .Bind<IKeyboardDevice>().ToConstant(s_keyboard)
                 .Bind<IPlatformSettings>().ToConstant(instance)
                 .Bind<IPlatformThreadingInterface>().ToConstant(instance)
                 .Bind<IRenderLoop>().ToConstant(new RenderLoop())