1. Create a new Unity project and open it
2. Right click on Project tab and add another Project tab (Add Tab --> Project)
3. Close the added Project tab and notice that no errors are thrown in the console
4. Turn on Collaborate, notice that a few Null Reference Exceptions are thrown in the console
5. Repeat steps 2 and 3

Expected result: No errors are thrown in the console
Actual result: Null Reference Exceptions are thrown in the console when closing/dragging/opening Project tab

Regression introduced with - 2017.1.0p3
Reproducible with - 2018.1.0a7, 2017.3.0f2, 2017.2.1f1, 2017.1.2p3, 2017.1.0p3
Not reproducible with - 2017.1.0p2

Notes:
1. Happens with other tabs as well, but the repro with the Project tab is the most consistent one
2. Seems that turning off Collab does not solve the issue and the errors keep re-appearing, but the errors do not happen before turning on Collab service