Steps to reproduce:

1. Create a new 3D URP project
2. In the Hierarchy, right-click > UI Toolkit  > Standard Elements and select anything
3. Save the .uxml and observe the UI Viewport window opens
4. Double click on UI Viewport tab to maximize it
5. Unmaximize it
6. Observe the Console

Actual results: "Invalid editor window of type: UnityEditor.FallbackEditorWindow, title: Failed to load" and "NullReferenceException: Object reference not set to an instance of an object" errors are thrown

Expected results: No errors are thrown

Reproducible with versions: 6000.6.0a7

Can’t test with versions: 6000.6.0a6 (UI Viewport window not available)

Tested on (OS): Windows 11

Full errors:
*
{code:java}
Invalid editor window of type: UnityEditor.FallbackEditorWindow, title: Failed to load
UnityEditor.EditorApplication:Internal_CallDelayFunctions (){code}

*
{code:java}
NullReferenceException: Object reference not set to an instance of an object
Unity.UIToolkit.Editor.UIViewportWindow.ReleaseStage (UnityEngine.EntityId stageId) (at <d347da126d734a6886395f445e0eee85>:0)
Unity.UIToolkit.Editor.UIViewportWindow.OnDestroy () (at <d347da126d734a6886395f445e0eee85>:0)
UnityEditor.GUIView:ProcessEvent(EntityId, IntPtr, Boolean&){code}