Steps to reproduce :

- create an animator controller
- create a sub state machine
- enter the sub state machine and create an empty state
- change the Inspector to Debug mode ( right click on the inspector tab )
- set the Parent State Machine to none
- drag the state on top of (Up)Base Layer and notice the crash

*Note :
Reproed on 4.2b1.

Explanation :
We don't support editing StateMachines with the debug inspector.