How to reproduce:

1. Open the “MinimalApp.zip” project
2. Open Edit → Project Settings → Editor, and ensure that the domain reload settings are set to "Reload Scene"
3. Create and open a new UXML file in UI Builder
4. Dock the UI Builder beside the Game view
5. Drag a new VisualElement into the UI Builder’s Hierarchy
6. Save the changes
7. Remove the VisualElement
8. Save the changes again
9. Focus on the Game view
10. Focus on the UI Builder
11. Observe the Console and the UI Builder

Actual results: No errors are thrown, UI Builder renders
Expected results: “NullReferenceException“ error is thrown, and UI Builder fails to render while it is docked

Reproducible in: 6000.0.0b16, 6000.0.63f1, 6000.2.14f1, 6000.3.0f1, 6000.4.0a5, 6000.5.0a2
Not reproducible in: 6000.0.0b15

Reproduced on: Windows Pro (25H2)
Not reproduced on: No other environment tested

Notes:
- Not reproducible if Reload Domain is enabled
- Reproducible only if the UI Builder window is docked on the same panel as the Game view
- After reproduction, UI Builder becomes usable only if it becomes a floating window
- Reproducible in a new UXML file as well