To reproduce this bug:

1. Create a new project
2. Press Assets -> Create -> Animator Controller
3. Double-click on created animator controller to open Animator window
4. Try to drag "Any State" block. It works fine
5. Press context button at the top-right corner of Animator window
6. Choose "Maximize" option. Animator window should be in docked state (not floating) to use this option
7. Try to drag "Any State". Still possible to drag it.
8. One more time press context button and uncheck "Maximize" option
9. If you try to drag "Any State" now yow will get NullReferenceException and it will be impossible to drag "AnyState"