[RENDER] Don't crash if UpdateScene produced a null scene

samples/ControlCatalog.NetCore/Program.cs

@@ -43,7 +43,7 @@ namespace ControlCatalog.NetCore
         /// This method is needed for IDE previewer infrastructure
         /// </summary>
         public static AppBuilder BuildAvaloniaApp()
-            => AppBuilder.Configure<App>().UsePlatformDetect().UseSkia().UseReactiveUI();
+            => AppBuilder.Configure<App>().UseX11().UseSkia().UseReactiveUI();
 
         static void ConsoleSilencer()
         {

src/Avalonia.Visuals/Rendering/DeferredRenderer.cs

@@ -152,7 +152,8 @@ namespace Avalonia.Rendering
             var t = (IRenderLoopTask)this;
             if(t.NeedsUpdate)
                 UpdateScene();
-            Render(true);
+            if(_scene.Item != null)
+                Render(true);
         }
 
         /// <inheritdoc/>