How to reproduce:
1. Open the attached project
2. Undock Game or Scene view
3. Enable verify saving assets (Edit - Preferences -> Verify Saving Assets)
4. Go to Window -> Text Mesh Pro - Font Asset Creator
5. Select Font Source (eg. Bangers)
6. Click on Generate Font Atlas
7. Click on Save textMeshPro font assets
8. When saving asset verification window appears click Save All

Actual Result: Notice NullReferenceException
Expected result: exception is not thrown

Reproducible: 5.3.6p8, 5.4.2p2, 5.5.0b5, 5.5.0b10

5.3, 5.4 versions throw InvalidOperationException and docking game or scene view doesn't solve the issue