I upgraded project from 2017.2.0b1 to 2017.2.0b3 and now I get Null Reff Exception on selecting prefab. If I open this project back in 2017.2.0b1 - prefab behaves ok.

How we can reproduce it using the example you attached
1 Open attached project
2 Open scene Offlinescene
3 Select "Capsule" prefab in Project view

Actual result:
Inspector is broken and errors appear:
- "ArgumentNullException: Argument cannot be null.
Parameter name: obj is nul"
- GUI Error: You are pushing more GUIClips than you are popping. Make sure they are balanced)

Expected results:
prefab should work ok