How to reproduce:
1. Open the attached “IN-104603” project
2. Open the “SampleScene”
3. Save the scene
4. Observe the Scene view and the Console

Expected result: The UI Document is rendered, no errors in the Console
Actual result: The UI Document is not rendered, “UnityException” error is thrown

Reproducible with: 6000.2.0a7, 6000.2.0b6
Not reproducible with: 6000.0.51f1, 6000.1.8f1, 6000.2.0a6
Couldn’t test with: 2021.3.53f1, 2022.3.63f1 (World space rendering not implemented)

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

Notes:
- An “UnityException: GetPtrFromInstanceID can only be called from the main thread.” error is thrown
- Error is repeatedly thrown while the “UIDocument” is in any Camera's view and moving the mouse cursor
- Only reproducible when Font asset is used in the Font property of the UXML