Steps to reproduce :

- import the project attached
- create a new animation controller
- create a new blend tree and double click it to edit it
- create a new blend tree to the existing one and a motion field
- add the "Run" animation to the motion field and notice that it overlaps the new blend tree
- click a few times between the motion field and the new blend tree that are overlapping and Unity will crash

*Note:
Regression from 4.6b5. The crash does not occur but the overlapping is still there.