diff --git a/src/Avalonia.Diagnostics/Diagnostics/DevTools.cs b/src/Avalonia.Diagnostics/Diagnostics/DevTools.cs
index 78e808595f..0e36c8f9cb 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/DevTools.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/DevTools.cs
@@ -22,7 +22,7 @@ namespace Avalonia.Diagnostics
 
         public static IDisposable Attach(TopLevel root, DevToolsOptions options)
         {
-            void PreviewKeyDown(object sender, KeyEventArgs e)
+            void PreviewKeyDown(object? sender, KeyEventArgs e)
             {
                 if (options.Gesture.Matches(e))
                 {
@@ -69,9 +69,9 @@ namespace Avalonia.Diagnostics
             return Disposable.Create(() => window?.Close());
         }
 
-        private static void DevToolsClosed(object sender, EventArgs e)
+        private static void DevToolsClosed(object? sender, EventArgs e)
         {
-            var window = (MainWindow)sender;
+            var window = (MainWindow)sender!;
             s_open.Remove(window.Root!);
             window.Closed -= DevToolsClosed;
         }
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewLocator.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewLocator.cs
index be3564e781..16852001da 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewLocator.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewLocator.cs
@@ -9,12 +9,12 @@ namespace Avalonia.Diagnostics
     {
         public IControl Build(object data)
         {
-            var name = data.GetType().FullName.Replace("ViewModel", "View");
+            var name = data.GetType().FullName!.Replace("ViewModel", "View");
             var type = Type.GetType(name);
 
             if (type != null)
             {
-                return (Control)Activator.CreateInstance(type);
+                return (Control)Activator.CreateInstance(type)!;
             }
             else
             {
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ClrPropertyViewModel.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ClrPropertyViewModel.cs
index 19e4a702eb..ab088a208c 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ClrPropertyViewModel.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ClrPropertyViewModel.cs
@@ -14,7 +14,7 @@ namespace Avalonia.Diagnostics.ViewModels
             _target = o;
             Property = property;
 
-            if (!property.DeclaringType.IsInterface)
+            if (property.DeclaringType == null || !property.DeclaringType.IsInterface)
             {
                 Name = property.Name;
             }
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlDetailsViewModel.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlDetailsViewModel.cs
index 74709cb91a..1f7205eb16 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlDetailsViewModel.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlDetailsViewModel.cs
@@ -250,7 +250,7 @@ namespace Avalonia.Diagnostics.ViewModels
                 .Select(x => new ClrPropertyViewModel(o, x));
         }
 
-        private void ControlPropertyChanged(object sender, AvaloniaPropertyChangedEventArgs e)
+        private void ControlPropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
         {
             if (_propertyIndex.TryGetValue(e.Property, out var properties))
             {
@@ -263,9 +263,10 @@ namespace Avalonia.Diagnostics.ViewModels
             Layout.ControlPropertyChanged(sender, e);
         }
 
-        private void ControlPropertyChanged(object sender, PropertyChangedEventArgs e)
+        private void ControlPropertyChanged(object? sender, PropertyChangedEventArgs e)
         {
-            if (_propertyIndex.TryGetValue(e.PropertyName, out var properties))
+            if (e.PropertyName != null
+                && _propertyIndex.TryGetValue(e.PropertyName, out var properties))
             {
                 foreach (var property in properties)
                 {
@@ -279,7 +280,7 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        private void OnClassesChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void OnClassesChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             if (!SnapshotStyles)
             {
@@ -351,10 +352,10 @@ namespace Avalonia.Diagnostics.ViewModels
         {
             public static PropertyComparer Instance { get; } = new PropertyComparer();
 
-            public int Compare(PropertyViewModel x, PropertyViewModel y)
+            public int Compare(PropertyViewModel? x, PropertyViewModel? y)
             {
-                var groupX = GroupIndex(x.Group);
-                var groupY = GroupIndex(y.Group);
+                var groupX = GroupIndex(x?.Group);
+                var groupY = GroupIndex(y?.Group);
 
                 if (groupX != groupY)
                 {
@@ -362,11 +363,11 @@ namespace Avalonia.Diagnostics.ViewModels
                 }
                 else
                 {
-                    return string.CompareOrdinal(x.Name, y.Name);
+                    return string.CompareOrdinal(x?.Name, y?.Name);
                 }
             }
 
-            private int GroupIndex(string group)
+            private int GroupIndex(string? group)
             {
                 switch (group)
                 {
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlLayoutViewModel.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlLayoutViewModel.cs
index f691698488..4dc0c34c0a 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlLayoutViewModel.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlLayoutViewModel.cs
@@ -179,7 +179,7 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        public void ControlPropertyChanged(object sender, AvaloniaPropertyChangedEventArgs e)
+        public void ControlPropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
         {
             try
             {
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/EventTreeNode.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/EventTreeNode.cs
index f6f57b769d..65fd81cc78 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/EventTreeNode.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/EventTreeNode.cs
@@ -59,14 +59,14 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        private void HandleEvent(object sender, RoutedEventArgs e)
+        private void HandleEvent(object? sender, RoutedEventArgs e)
         {
             if (!_isRegistered || IsEnabled == false)
                 return;
             if (sender is IVisual v && BelongsToDevTool(v))
                 return;
 
-            var s = sender;
+            var s = sender!;
             var handled = e.Handled;
             var route = e.Route;
 
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/FilterViewModel.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/FilterViewModel.cs
index e592c89892..5b27236f2e 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/FilterViewModel.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/FilterViewModel.cs
@@ -110,9 +110,10 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        public IEnumerable GetErrors(string propertyName)
+        public IEnumerable GetErrors(string? propertyName)
         {
-            if (_errors.TryGetValue(propertyName, out var error))
+            if (propertyName != null
+                && _errors.TryGetValue(propertyName, out var error))
             {
                 yield return error;
             }
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/MainViewModel.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/MainViewModel.cs
index 4a945c253f..98afafe7d4 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/MainViewModel.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/MainViewModel.cs
@@ -190,7 +190,7 @@ namespace Avalonia.Diagnostics.ViewModels
             FocusedControl = KeyboardDevice.Instance.FocusedElement?.GetType().Name;
         }
 
-        private void KeyboardPropertyChanged(object sender, PropertyChangedEventArgs e)
+        private void KeyboardPropertyChanged(object? sender, PropertyChangedEventArgs e)
         {
             if (e.PropertyName == nameof(KeyboardDevice.Instance.FocusedElement))
             {
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/PropertyViewModel.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/PropertyViewModel.cs
index 779d74d554..bfd098985a 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/PropertyViewModel.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/PropertyViewModel.cs
@@ -31,13 +31,13 @@ namespace Avalonia.Diagnostics.ViewModels
             if (!converter.CanConvertTo(typeof(string)) || 
                 converter.GetType() == typeof(CollectionConverter))
             {
-                return value.ToString();
+                return value.ToString() ?? "(null)";
             }
 
             return converter.ConvertToString(value);
         }
 
-        private static object InvokeParse(string s, Type targetType)
+        private static object? InvokeParse(string s, Type targetType)
         {
             var method = targetType.GetMethod("Parse", PublicStatic, null, StringIFormatProviderParameters, null);
 
@@ -56,7 +56,7 @@ 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)
         {
             var converter = TypeDescriptor.GetConverter(targetType);
 
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ResourceSetterViewModel.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ResourceSetterViewModel.cs
index 2655b0e31f..e93dc7361b 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ResourceSetterViewModel.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ResourceSetterViewModel.cs
@@ -16,12 +16,14 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public void CopyResourceKey()
         {
-            if (Key is null)
+            var textToCopy = Key?.ToString();
+
+            if (textToCopy is null)
             {
                 return;
             }
 
-            CopyToClipboard(Key.ToString());
+            CopyToClipboard(textToCopy);
         }
     }
 }
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/SetterViewModel.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/SetterViewModel.cs
index 1f0e93d751..38cbefcb93 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/SetterViewModel.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/SetterViewModel.cs
@@ -36,12 +36,14 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public void CopyValue()
         {
-            if (Value is null)
+            var textToCopy = Value?.ToString();
+
+            if (textToCopy is null)
             {
                 return;
             }
 
-            CopyToClipboard(Value.ToString());
+            CopyToClipboard(textToCopy);
         }
 
         public void CopyPropertyName()
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreeNodeCollection.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreeNodeCollection.cs
index e01258f4cc..06a6fa9a74 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreeNodeCollection.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreeNodeCollection.cs
@@ -35,7 +35,7 @@ namespace Avalonia.Diagnostics.ViewModels
 
         protected TreeNode Owner { get; }
 
-        public event NotifyCollectionChangedEventHandler CollectionChanged
+        public event NotifyCollectionChangedEventHandler? CollectionChanged
         {
             add
             {
@@ -49,7 +49,7 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        public event PropertyChangedEventHandler PropertyChanged
+        public event PropertyChangedEventHandler? PropertyChanged
         {
             add
             {
diff --git a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ViewModelBase.cs b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ViewModelBase.cs
index 0bc44d5778..a2ee37c625 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ViewModelBase.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/ViewModels/ViewModelBase.cs
@@ -10,7 +10,7 @@ namespace Avalonia.Diagnostics.ViewModels
         private PropertyChangedEventHandler? _propertyChanged;
         private List<string> events = new List<string>();
 
-        public event PropertyChangedEventHandler PropertyChanged
+        public event PropertyChangedEventHandler? PropertyChanged
         {
             add { _propertyChanged += value; events.Add("added"); }
             remove { _propertyChanged -= value; events.Add("removed"); }
diff --git a/src/Avalonia.Diagnostics/Diagnostics/Views/ConsoleView.xaml.cs b/src/Avalonia.Diagnostics/Diagnostics/Views/ConsoleView.xaml.cs
index a018e4180e..ab523fb75a 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/Views/ConsoleView.xaml.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/Views/ConsoleView.xaml.cs
@@ -30,15 +30,15 @@ namespace Avalonia.Diagnostics.Views
             AvaloniaXamlLoader.Load(this);
         }
 
-        private void HistoryChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void HistoryChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
-            if (e.Action == NotifyCollectionChangedAction.Add && e.NewItems[0] is IControl control)
+            if (e.Action == NotifyCollectionChangedAction.Add && e.NewItems?[0] is IControl control)
             {
                 DispatcherTimer.RunOnce(control.BringIntoView, TimeSpan.Zero);
             }
         }
 
-        private void InputKeyDown(object sender, KeyEventArgs e)
+        private void InputKeyDown(object? sender, KeyEventArgs e)
         {
             var vm = (ConsoleViewModel?)DataContext;
             if (vm is null)
diff --git a/src/Avalonia.Diagnostics/Diagnostics/Views/EventsPageView.xaml.cs b/src/Avalonia.Diagnostics/Diagnostics/Views/EventsPageView.xaml.cs
index 687a20c5f6..ba7ab41e35 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/Views/EventsPageView.xaml.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/Views/EventsPageView.xaml.cs
@@ -53,7 +53,7 @@ namespace Avalonia.Diagnostics.Views
             }
         }
 
-        private void OnRecordedEventsChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void OnRecordedEventsChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             if (sender is ObservableCollection<FiredEvent> events)
             {
diff --git a/src/Avalonia.Diagnostics/Diagnostics/Views/MainView.xaml.cs b/src/Avalonia.Diagnostics/Diagnostics/Views/MainView.xaml.cs
index f30973fea9..b688ad7676 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/Views/MainView.xaml.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/Views/MainView.xaml.cs
@@ -58,7 +58,7 @@ namespace Avalonia.Diagnostics.Views
             AvaloniaXamlLoader.Load(this);
         }
 
-        private void PreviewKeyDown(object sender, KeyEventArgs e)
+        private void PreviewKeyDown(object? sender, KeyEventArgs e)
         {
             if (e.Key == Key.Escape)
             {
diff --git a/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs b/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs
index fdadf114c9..bbb8e76551 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs
@@ -114,6 +114,6 @@ namespace Avalonia.Diagnostics.Views
             }
         }
 
-        private void RootClosed(object sender, EventArgs e) => Close();
+        private void RootClosed(object? sender, EventArgs e) => Close();
     }
 }
diff --git a/src/Avalonia.Diagnostics/Diagnostics/Views/TreePageView.xaml.cs b/src/Avalonia.Diagnostics/Diagnostics/Views/TreePageView.xaml.cs
index 6ad6533e7a..3543b1adea 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/Views/TreePageView.xaml.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/Views/TreePageView.xaml.cs
@@ -38,9 +38,9 @@ namespace Avalonia.Diagnostics.Views
             };
         }
 
-        protected void AddAdorner(object sender, PointerEventArgs e)
+        protected void AddAdorner(object? sender, PointerEventArgs e)
         {
-            var node = (TreeNode?)((Control)sender).DataContext;
+            var node = (TreeNode?)((Control)sender!).DataContext;
             var vm = (TreePageViewModel?)DataContext;
             if (node is null || vm is null)
             {
@@ -79,7 +79,7 @@ namespace Avalonia.Diagnostics.Views
             return new Thickness(-input.Left, -input.Top, -input.Right, -input.Bottom);
         }
 
-        protected void RemoveAdorner(object sender, PointerEventArgs e)
+        protected void RemoveAdorner(object? sender, PointerEventArgs e)
         {
             foreach (var border in _adorner.Children.OfType<Border>())
             {
@@ -97,15 +97,15 @@ namespace Avalonia.Diagnostics.Views
             AvaloniaXamlLoader.Load(this);
         }
 
-        private void TreeViewItemMaterialized(object sender, ItemContainerEventArgs e)
+        private void TreeViewItemMaterialized(object? sender, ItemContainerEventArgs e)
         {
             var item = (TreeViewItem)e.Containers[0].ContainerControl;
             item.TemplateApplied += TreeViewItemTemplateApplied;
         }
 
-        private void TreeViewItemTemplateApplied(object sender, TemplateAppliedEventArgs e)
+        private void TreeViewItemTemplateApplied(object? sender, TemplateAppliedEventArgs e)
         {
-            var item = (TreeViewItem)sender;
+            var item = (TreeViewItem)sender!;
 
             // This depends on the default tree item template.
             // We want to handle events in the item header but exclude events coming from children.