fixes(DevTools): NRE when click 'Visualizze dirty rects' and 'Show fps overly' in Option menu

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

@@ -28,7 +28,12 @@ namespace Avalonia.Diagnostics.Controls
                 };
                 controller.Exit += eh;
             }
-
+            RendererRoot = application.ApplicationLifetime switch
+            {
+                Lifetimes.IClassicDesktopStyleApplicationLifetime classic => classic.MainWindow.Renderer,
+                Lifetimes.ISingleViewApplicationLifetime single => (single.MainView as VisualTree.IVisual)?.VisualRoot?.Renderer,
+                _ => null
+            };
         }
 
         internal App Instance => _application;
@@ -105,5 +110,10 @@ namespace Avalonia.Diagnostics.Controls
         /// </summary>
         public string? Name =>
             _application.Name;
+
+        /// <summary>
+        /// Gets the root of the visual tree, if the control is attached to a visual tree.
+        /// </summary>
+        internal Rendering.IRenderer? RendererRoot { get; }
     }
 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/MainViewModel.cs

@@ -76,7 +76,20 @@ namespace Avalonia.Diagnostics.ViewModels
             get => _shouldVisualizeDirtyRects;
             set
             {
-                ((TopLevel)_root).Renderer.DrawDirtyRects = value;
+                var changed = true;
+                if (_root is TopLevel topLevel && topLevel.Renderer is { })
+                {
+                    topLevel.Renderer.DrawDirtyRects = value;
+                }
+                else if (_root is Controls.Application app && app.RendererRoot is { })
+                {
+                    app.RendererRoot.DrawDirtyRects = value;
+                }
+                else
+                {
+                    changed = false;
+                }
+                if (changed)
                 RaiseAndSetIfChanged(ref _shouldVisualizeDirtyRects, value);
             }
         }
@@ -96,8 +109,21 @@ namespace Avalonia.Diagnostics.ViewModels
             get => _showFpsOverlay;
             set
             {
-                ((TopLevel)_root).Renderer.DrawFps = value;
-                RaiseAndSetIfChanged(ref _showFpsOverlay, value);
+                var changed = true;
+                if (_root is TopLevel topLevel && topLevel.Renderer is { })
+                {
+                    topLevel.Renderer.DrawFps = value;
+                }
+                else if (_root is Controls.Application app && app.RendererRoot is { })
+                {
+                    app.RendererRoot.DrawFps = value;
+                }
+                else
+                {
+                    changed = false;
+                }
+                if(changed)
+                    RaiseAndSetIfChanged(ref _showFpsOverlay, value);
             }
         }