How to reproduce:
1. Open any project
2. Create a Prefab
3. In the Hierarchy window, select the created Prefab
4. Click the arrow “Open Prefab Asset in context”
5. In the Inspector window, click the lock icon to lock the window
6. In the Hierarchy window, click the arrow to exit the “Prefab Asset in context” mode
7. Enter Play mode
8. Observe the console
Actual result: Exception “SerializedObjectNotCreatableException: Object at index 0 is null” is thrown
Expected result: No exceptions are thrown
Reproducible with: 6000.0.73f1, 6000.1.0a3, 6000.3.14f1, 6000.4.5f1, 6000.5.0a4
Not reproducible with: 6000.1.0a2, 6000.5.0b6, 6000.6.0a4
Fixed in: 6000.5.0a5
Reproducible on: macOS 26.4.1 (by user), Windows 11
Not reproducible on: No other environments tested
Note: The error is clearable but appears every time you enter Play mode until you restart the Editor