Steps to reproduce:
1. Create a Behavior Graph
2. Open it and dock it somewhere - and change focus
3. Delete it outside of the editor
4. Observe that the behavior window is still open
5. Cause a domain reload (will cause the managed code to GC)
6. Enter Playmode

Actual results: 

• nullrefence is raised on enter playmode.
• trying to close the behavior window crash the editor layout and require a restart.

Expected results: 

• Behavior window is close automatically if the asset no longer exist.