To Reproduce:

1. Open the attached project & select the sphere object.
2. Lock inspector & play.
3. Observe 'the MissingReferenceException'
4. The Inspector tab is now permanently broken:
-it doesn't display anything
-if you have other tabs in the same window you can't select them.
-you can't unlock it
-the null ref error is thrown after doing anything
You have to reopen the editor to fix it.

Only happens on objects with custom types. Adding a new script to an object which has it's inspector locked, triggers the same nullref error, but the inspector remains usable.

Can only repro on Alpha 19