More nullability fixes.

src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreePageViewModel.cs

@@ -86,14 +86,15 @@ namespace Avalonia.Diagnostics.ViewModels
         public void SelectControl(IControl control)
         {
             var node = default(TreeNode);
+            IControl? c = control;
 
-            while (node == null && control != null)
+            while (node == null && c != null)
             {
-                node = FindNode(control);
+                node = FindNode(c);
 
                 if (node == null)
                 {
-                    control = control.GetVisualParent<IControl>();
+                    c = c.GetVisualParent<IControl>();
                 }
             }
 

src/Avalonia.Input/AccessKeyHandler.cs

@@ -188,7 +188,7 @@ namespace Avalonia.Input
                 // If the menu is open, only match controls in the menu's visual tree.
                 if (menuIsOpen)
                 {
-                    matches = matches.Where(x => MainMenu.IsVisualAncestorOf(x));
+                    matches = matches.Where(x => x is not null && MainMenu!.IsVisualAncestorOf(x));
                 }
 
                 var match = matches.FirstOrDefault();

src/Avalonia.Input/InputElement.cs

@@ -675,7 +675,7 @@ namespace Avalonia.Input
         /// <see cref="IsEffectivelyEnabled"/>.
         /// </summary>
         /// <param name="parent">The parent control.</param>
-        private void UpdateIsEffectivelyEnabled(InputElement parent)
+        private void UpdateIsEffectivelyEnabled(InputElement? parent)
         {
             IsEffectivelyEnabled = IsEnabledCore && (parent?.IsEffectivelyEnabled ?? true);
 

src/Avalonia.Input/Pointer.cs

@@ -54,7 +54,7 @@ namespace Avalonia.Input
                 Captured.DetachedFromVisualTree += OnCaptureDetached;
         }
 
-        IInputElement GetNextCapture(IVisual parent)
+        IInputElement? GetNextCapture(IVisual parent)
         {
             return parent as IInputElement ?? parent.FindAncestorOfType<IInputElement>();
         }

src/Web/Avalonia.Web.Blazor/RazorViewTopLevelImpl.cs

@@ -106,8 +106,8 @@ namespace Avalonia.Web.Blazor
 
         public IRenderer CreateRenderer(IRenderRoot root)
         {
-            var loop = AvaloniaLocator.Current.GetService<IRenderLoop>();
-
+            var loop = AvaloniaLocator.Current.GetService<IRenderLoop>() ??
+                throw new InvalidOperationException("Unable to locate IRenderLoop.");
             return new DeferredRenderer(root, loop);
         }