How to reproduce:
1. Open the user-attached project “UxmlObjectReferences_override”
2. Open the “Doc” Asset in the Project window
3. In the UI Builder Hierarchy expand the “TemplateContainer” Template Instance
4. Open the “EditMe” Element in the UI Builder Inspector
5. Under the “Attributes” section, click “Add”
6. Observe the Console

Expected results: There are no errors thrown
Actual results: “NullReferenceException” error is thrown into the Console

Reproducible with: 6000.0.4f1
Can’t test with: 2021.3.38f1, 2022.3.31f1 (Script can’t be loaded)

Reproducible on: Windows 11 Pro, macOS 14.4.1 (user’s)
Not reproducible on: No other environment tested