[Collab] Crash when switching from Unity collab project on the disk, to cached project from collab cloud

Steps to reproduce :

1) Create a new project with a test scene. Switch on the Unity service "Collaborate".
2) Open a new cached project from the collab cloud.
3) Observe the crash

Note: Please see attached screenshots to reproduce the bug.

Reproduced On: 2017.3.0b4 , 2018.1.0a4

Works fine On: 2017.2.0f3, 2017.1.0f3

Regression Introduced On: 2017.3.0b4

License type: Pro

Platform: Windows 10