Editor crashes when assigning the currently-edited UXML to a Column's "Header" Visual Tree Asset field in MultiColumnListView's Columns list

Steps to reproduce:

1. Create a new Unity project using the Universal 3D template
2. Open the UI Library (Right-click in the Hierarchy > UI Toolkit > Standard Elements > UI Library...)
3. Double click the "Multi Column List View"
4. Click the "Continue" button
5. Click the "Save" button
6. Select the "MultiColumnListView" in the Hierarchy
7. Open the "Columns" foldout in the Inspector
8. Click the "+" button to add a new element
9. Assign the same UXML Document to the "Header Template" field
10. Observe the Editor

Actual results: The Editor crashes due to circular dependency, no error dialog, no console error, unsaved changes to the document are lost.

Expected results: The assignment is rejected - the currently-edited document should either be excluded from the object picker, or the assignment should be blocked with a circular-dependency warning

Reproducible with versions: 6000.6.0f1, 6000.7.0a6

Can’t test with versions: 6000.0.0f1, 6000.0.82f1, 6000.3.23f1, 6000.5.11f1 (due to no UITK editing stage feature)

Tested on (OS): MacOS Silicon Tahoe 26.6.2

Notes:

• When reproduced on 6000.6.0f1 the Bug Reporter does not spawn
• Does not reproduce in UI Builder (has circular dependency warning popup)
• Does not reproduce in the "Cell Template" field (doesn't crash, but no circular dependency popup and the UXML Document gets assigned)