Errors are thrown when selecting an Editor Extension Authoring enabled UXML after exiting Play Mode

Steps to reproduce:

1. Create and open a new project
2. Create and open a new UXML file in UI Builder
3. Enable the "Editor Extension Authoring" on the root VisualElement
4. Save the changes and close the window
5. Attach the UXML to a UI Document in the Scene
6. Enter and Exit Play Mode
7. Select the UXML file
8. Observe the Console

Actual results: Nothing is thrown

Expected results: A NullReferenceException and an "ArgumentException: Getting control 1's position in a group with only 1 controls when doing repaint" errors are thrown

Reproducible with versions: 6000.5.0a4, 6000.5.0b10, 6000.6.0a6

Not reproducible with versions: 6000.0.76f1, 6000.3.16f1, 6000.4.9f1, 6000.5.0a3

Tested on (OS): Windows 11 Pro (25H2)

Notes:

• The new Hierarchy window has to be enabled for the reproduction