Steps to repro:

1. Open attached project and '9_Realtime GI' scene;
2. Go to Lighting settings (Window -> Lighting -> Settings);
3. In Lighmapping Settings switch Lightmapper to PVR.

Expected result:
No errors are produced.

Actual result:
Error:
'Assertion failed: Assertion failed on expression: 'Thread::EqualsCurrentThreadIDForAssert(Thread::mainThreadId)''

is shown in the console. See attached video.

Regression introduced in Unity 2017.1.0b8.

Notes:
- Reproducible in 2017.1.0b8;
- Not reproducible in 2017.2.0a1, 5.6.1p2, 5.5.2p1;
- Reproducible on Windows 10 and OSX 10.12.5;
- The issue seems to be scene-specific; it doesn't repro in the new scene with primitive gameobjects.
- The issue happens only when switching from Enlighten to PVR and not vice versa;
- The issue also happen when starting baking lightmaps under PVR GI backend.