Fix nullability error from Avalonia.Web.Blazor.

src/Avalonia.Controls/ApplicationLifetimes/ISingleViewApplicationLifetime.cs

@@ -2,6 +2,6 @@ namespace Avalonia.Controls.ApplicationLifetimes
 {
     public interface ISingleViewApplicationLifetime : IApplicationLifetime
     {
-        Control MainView { get; set; }
+        Control? MainView { get; set; }
     }
 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/LogicalTreeNode.cs

@@ -67,7 +67,8 @@ namespace Avalonia.Diagnostics.ViewModels
 
             protected override void Initialize(AvaloniaList<TreeNode> nodes)
             {
-                if (_application.ApplicationLifetime is Lifetimes.ISingleViewApplicationLifetime single)
+                if (_application.ApplicationLifetime is Lifetimes.ISingleViewApplicationLifetime single &&
+                    single.MainView is not null)
                 {
                     nodes.Add(new LogicalTreeNode(single.MainView, Owner));
                 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/VisualTreeNode.cs

@@ -167,7 +167,8 @@ namespace Avalonia.Diagnostics.ViewModels
 
             protected override void Initialize(AvaloniaList<TreeNode> nodes)
             {
-                if (_application.ApplicationLifetime is Lifetimes.ISingleViewApplicationLifetime single)
+                if (_application.ApplicationLifetime is Lifetimes.ISingleViewApplicationLifetime single &&
+                    single.MainView is not null)
                 {
                     nodes.Add(new VisualTreeNode(single.MainView, Owner));
                 }

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

@@ -135,7 +135,7 @@ namespace Avalonia.Web.Blazor
 
         public PixelPoint PointToScreen(Point point) => new PixelPoint((int)point.X, (int)point.Y);
 
-        public void SetCursor(ICursorImpl cursor)
+        public void SetCursor(ICursorImpl? cursor)
         {
             var val = (cursor as CssCursor)?.Value ?? CssCursor.Default;
             if (_currentCursor != val)