[RENDER] Don't crash on Paint call when renderer is already disposed

src/Avalonia.Visuals/Rendering/DeferredRenderer.cs

@@ -29,6 +29,7 @@ namespace Avalonia.Rendering
         private readonly ISceneBuilder _sceneBuilder;
 
         private bool _running;
+        private bool _disposed;
         private volatile IRef<Scene> _scene;
         private DirtyVisuals _dirty;
         private IRef<IRenderTargetBitmapImpl> _overlay;
@@ -125,6 +126,9 @@ namespace Avalonia.Rendering
         {
             lock (_sceneLock)
             {
+                if (_disposed)
+                    return;
+                _disposed = true;
                 var scene = _scene;
                 _scene = null;
                 scene?.Dispose();
@@ -171,7 +175,7 @@ namespace Avalonia.Rendering
             var t = (IRenderLoopTask)this;
             if(t.NeedsUpdate)
                 UpdateScene();
-            if(_scene.Item != null)
+            if(_scene?.Item != null)
                 Render(true);
         }
 
@@ -481,6 +485,8 @@ namespace Avalonia.Rendering
             Dispatcher.UIThread.VerifyAccess();
             lock (_sceneLock)
             {
+                if (_disposed)
+                    return;
                 if (_scene?.Item.Generation > _lastSceneId)
                     return;
             }