AssertionException: Expected: not null But was: null, when use Blend Tree

2) How can we reproduce it using the example you attached

1 Create new project
2 Create Animator controller and open it
3 Create Blend Tree and open it (you should be on Blend Tree layer)
4 Close Editor without saving
5 Open project again
- Animtor should be opened by default
6 Go to Base Layer and notice exception in the console

AssertionException: Expected: not null
But was: null

Please see attached video for steps 5,6