Fix up nullable issues in Avalonia.Diagnostics.

src/Avalonia.Diagnostics/Diagnostics/Controls/Application.cs

@@ -30,7 +30,7 @@ namespace Avalonia.Diagnostics.Controls
             }
             RendererRoot = application.ApplicationLifetime switch
             {
-                Lifetimes.IClassicDesktopStyleApplicationLifetime classic => classic.MainWindow.Renderer,
+                Lifetimes.IClassicDesktopStyleApplicationLifetime classic => classic.MainWindow?.Renderer,
                 Lifetimes.ISingleViewApplicationLifetime single => (single.MainView as VisualTree.IVisual)?.VisualRoot?.Renderer,
                 _ => null
             };

src/Avalonia.Diagnostics/Diagnostics/ViewLocator.cs

@@ -7,8 +7,11 @@ namespace Avalonia.Diagnostics
 {
     internal class ViewLocator : IDataTemplate
     {
-        public IControl Build(object data)
+        public IControl? Build(object? data)
         {
+            if (data is null)
+                return null;
+
             var name = data.GetType().FullName!.Replace("ViewModel", "View");
             var type = Type.GetType(name);
 
@@ -22,7 +25,7 @@ namespace Avalonia.Diagnostics
             }
         }
 
-        public bool Match(object data)
+        public bool Match(object? data)
         {
             return data is ViewModelBase;
         }

src/Avalonia.Diagnostics/Diagnostics/Views/ConsoleView.xaml.cs

@@ -17,9 +17,9 @@ namespace Avalonia.Diagnostics.Views
         public ConsoleView()
         {
             this.InitializeComponent();
-            _historyList = this.FindControl<ListBox>("historyList");
+            _historyList = this.GetControl<ListBox>("historyList");
             ((ILogical)_historyList).LogicalChildren.CollectionChanged += HistoryChanged;
-            _input = this.FindControl<TextBox>("input");
+            _input = this.GetControl<TextBox>("input");
             _input.KeyDown += InputKeyDown;
         }
 
@@ -54,12 +54,12 @@ namespace Avalonia.Diagnostics.Views
                     break;
                 case Key.Up:
                     vm.HistoryUp();
-                    _input.CaretIndex = _input.Text.Length;
+                    _input.CaretIndex = _input.Text?.Length ?? 0;
                     e.Handled = true;
                     break;
                 case Key.Down:
                     vm.HistoryDown();
-                    _input.CaretIndex = _input.Text.Length;
+                    _input.CaretIndex = _input.Text?.Length ?? 0;
                     e.Handled = true;
                     break;
             }

src/Avalonia.Diagnostics/Diagnostics/Views/EventsPageView.xaml.cs

@@ -19,7 +19,7 @@ namespace Avalonia.Diagnostics.Views
         public EventsPageView()
         {
             InitializeComponent();
-            _events = this.FindControl<ListBox>("EventsList");
+            _events = this.GetControl<ListBox>("EventsList");
         }
 
         public void NavigateTo(object sender, TappedEventArgs e)

src/Avalonia.Diagnostics/Diagnostics/Views/LayoutExplorerView.axaml.cs

@@ -24,20 +24,20 @@ namespace Avalonia.Diagnostics.Views
         {
             InitializeComponent();
 
-            _borderArea = this.FindControl<ThicknessEditor>("BorderArea");
-            _paddingArea = this.FindControl<ThicknessEditor>("PaddingArea");
+            _borderArea = this.GetControl<ThicknessEditor>("BorderArea");
+            _paddingArea = this.GetControl<ThicknessEditor>("PaddingArea");
 
-            _horizontalSizeBegin = this.FindControl<Rectangle>("HorizontalSizeBegin");
-            _horizontalSizeEnd = this.FindControl<Rectangle>("HorizontalSizeEnd");
-            _verticalSizeBegin = this.FindControl<Rectangle>("VerticalSizeBegin");
-            _verticalSizeEnd = this.FindControl<Rectangle>("VerticalSizeEnd");
+            _horizontalSizeBegin = this.GetControl<Rectangle>("HorizontalSizeBegin");
+            _horizontalSizeEnd = this.GetControl<Rectangle>("HorizontalSizeEnd");
+            _verticalSizeBegin = this.GetControl<Rectangle>("VerticalSizeBegin");
+            _verticalSizeEnd = this.GetControl<Rectangle>("VerticalSizeEnd");
 
-            _horizontalSize = this.FindControl<Border>("HorizontalSize");
-            _verticalSize = this.FindControl<Border>("VerticalSize");
+            _horizontalSize = this.GetControl<Border>("HorizontalSize");
+            _verticalSize = this.GetControl<Border>("VerticalSize");
 
-            _contentArea = this.FindControl<Border>("ContentArea");
+            _contentArea = this.GetControl<Border>("ContentArea");
 
-            _layoutRoot = this.FindControl<Grid>("LayoutRoot");
+            _layoutRoot = this.GetControl<Grid>("LayoutRoot");
 
             void SubscribeToBounds(Visual visual)
             {

src/Avalonia.Diagnostics/Diagnostics/Views/MainView.xaml.cs

@@ -19,9 +19,9 @@ namespace Avalonia.Diagnostics.Views
         {
             InitializeComponent();
             AddHandler(KeyDownEvent, PreviewKeyDown, RoutingStrategies.Tunnel);
-            _console = this.FindControl<ConsoleView>("console");
-            _consoleSplitter = this.FindControl<GridSplitter>("consoleSplitter");
-            _rootGrid = this.FindControl<Grid>("rootGrid");
+            _console = this.GetControl<ConsoleView>("console");
+            _consoleSplitter = this.GetControl<GridSplitter>("consoleSplitter");
+            _rootGrid = this.GetControl<Grid>("rootGrid");
             _consoleRow = Grid.GetRow(_console);
         }
 

src/Avalonia.Diagnostics/Diagnostics/Views/TreePageView.xaml.cs

@@ -20,8 +20,8 @@ namespace Avalonia.Diagnostics.Views
         public TreePageView()
         {
             InitializeComponent();
-            _tree = this.FindControl<TreeView>("tree");
-            _tree.ItemContainerGenerator.Index.Materialized += TreeViewItemMaterialized;
+            _tree = this.GetControl<TreeView>("tree");
+            _tree.ItemContainerGenerator.Index!.Materialized += TreeViewItemMaterialized;
 
             _adorner = new Panel
             {