Reproduction steps:
1. Open the attached project "ReproProj"
2. Open Window > UI Toolkit > Debugger
3. Dock the “UI Toolkit Debugger” tab next to another panel (e.g., dock it with the Inspector).
4. Click another tab (e.g., “Inspector”) so that “UI Toolkit Debugger” is docked but not the visible/active tab.
5. Close the Editor
6. Reopen the same project

Expected result: No Errors exist
Actual result: Error “NullReferenceException: Object reference not set to an instance of an object“ is present

Reproducible with: 6000.0.57f1, 6000.2.3f1, 6000.3.0a6

Testing environment: Windows 10 Enterprise 21H2
Not reproducible on: No other environment tested

Notes:
- The exception spam immediately stops once the “UI Toolkit Debugger” tab is brought to the foreground (made the active/visible tab)
- The issue does not occur if the UI Toolkit Debugger window is closed before launch, or if it is the active/visible tab when the Editor opens.