After adding new tree to the project - unable to close Unity

2) How can we reproduce it using the example you attached
Regression (started since 5.0.0a18, and not present in 5.0.0a17)
also reproduces in latest 5.0.0b1
Steps to reproduce:

1. Create new project
2. Add tree (main menu -> Game Object -> 3D Object -> Tree)
3. Try to close Unity (press "Don't save" in pop up window)
Actual result: Editor doesn't close itself, and after some time it crashes