How to reproduce:
1. Download the attached “BrokenLayout“ file and place it inside the Layouts folder (%Appdata%\Roaming\Unity\Editor-5.x\Preferences\Layouts\default)
2. Open the attached “IN-136298“ project
3. Open the Layout dropdown in the Editor toolbar and select “BrokenLayout“
4. Click on the Project window
5. Observe the Console

Actual result: NullReferenceException errors are present
Expected result: No errors are present

Reproducible with: 2023.1.0a1, 6000.0.70f1, 6000.3.11f1, 6000.4.0b12, 6000.5.0a8

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

Workaround: Do not save Layouts with locked Project Browsers

Notes:
- Clicking an existing element in the Project Browser stops the errors
- Clicking an existing element is sometimes not possible, as the Project Browser can stop responding. In this case a restart of the Editor is required