Steps to repro:

1. Open Unity;
2. Open attached project and 'test' scene;
3. Select Terrain gameobject in the Hierarchy view;
4. Go to 'Paint Details' tab;
5. Double click on 'Cube' detail mesh prototype;
6. Close appeared popup;
7. Select any other gameobject in the scene (e.g. 'Main Camera');

8. Notice that 'Destroying gameobject "" is not allowed at this time." error is shown;

9. Select Terrain gameobject in the Hierarchy view and delete it.

Actual result:
- Scene/Game views become completely unresponsive;
- Error:

'MissingReferenceException: The object of type 'Terrain' has been destroyed but you are still trying to access it.'

is thrown in the Console. See attached video.

Regression introduced in Unity 5.6.0b4

Notes:
- Reproducible in 5.6.0b5; 5.6.0b4; Not reproducible in 5.6.0b3, 5.5.0p4, 5.4.4p1;
- Reproducible on both OSX 10.12.2 and Windows 10;
- Also reproducible for undoing Terrain creation after having received 'Destroying gameobject "" is not allowed at this time." error.