Fix errors on Editor shutdown

Source/Editor/Windows/EditorOptionsWindow.cs

@@ -310,6 +310,8 @@ namespace FlaxEditor.Windows
 
         private void OnSelectedTabChanged(Tabs tabs)
         {
+            if (IsDisposing)
+                return;
             ApplySearchFilter();
         }
 

Source/Engine/UI/GUI/WindowRootControl.cs

@@ -119,7 +119,8 @@ namespace FlaxEngine.GUI
         /// <param name="force">True if move to the front by force, otherwise false.</param>
         public void BringToFront(bool force = false)
         {
-            _window.BringToFront(force);
+            if (_window && _window.IsVisible)
+                _window.BringToFront(force);
         }
 
         /// <summary>