Don't invalidate a disposed TopLevelImpl.

src/Avalonia.Controls/Embedding/Offscreen/OffscreenTopLevelImpl.cs

@@ -11,11 +11,13 @@ namespace Avalonia.Controls.Embedding.Offscreen
     {
         private double _scaling = 1;
         private Size _clientSize;
+
         public IInputRoot InputRoot { get; private set; }
+        public bool IsDisposed { get; private set; }
 
         public virtual void Dispose()
         {
-            //No-op
+            IsDisposed = true;
         }
 
         public IRenderer CreateRenderer(IRenderRoot root) => new ImmediateRenderer(root);

src/Avalonia.Controls/Remote/Server/RemoteServerTopLevelImpl.cs

@@ -331,8 +331,11 @@ namespace Avalonia.Controls.Remote.Server
 
         public override void Invalidate(Rect rect)
         {
-            _invalidated = true;
-            Dispatcher.UIThread.Post(RenderIfNeeded);
+            if (!IsDisposed)
+            {
+                _invalidated = true;
+                Dispatcher.UIThread.Post(RenderIfNeeded);
+            }
         }
 
         public override IMouseDevice MouseDevice { get; } = new MouseDevice();