Mesh had invalid face indices (number too high).
On Separate in Edit Mode it crashed.
Two fixes:
- The Material properties viewer just showed the last material in the index array.
Now it shows nothing, to indicate it's an invalid selected material.
- Crash was caused by array copy magic, not checking the active index properly.
(No assert, no warning prints, I think cases like this can happen too easily, and this
way user gets notified nicely and can fix it).
7b938abe2b