Reproduction steps:
1. Open the attached "IN-90075" project
2. In the Project window Assets folder double click on the “BaseDocument.uxml” file
3. In the UI Builder Hierarchy window select “MyElement”
4. Observe the Console window

Expected result: No errors appear
Actual result: A NullReferenceException error appears in the Console

Reproducible with: 2023.1.0a23, 6000.0.36f1, 6000.1.0b5, 6000.2.0a2
Not reproducible with: 2021.3.48f1, 2022.3.57f1, 2023.1.0a22

Reproducible on: Windows 11
Not reproducible on: No other environments tested