Preconditions:
For easiness of reproduction find some project in the organization or with a lot of changes you don't have.

To repro:
1. Open project connected to Collab and open Collab History window.
2. While it's still loading, go to File -> Open project -> open another project.

Actual result: Unity crashes with stacktrace:

Thread 0 Crashed:: CrBrowserMain Dispatch queue: com.apple.main-thread
0 libsystem_kernel.dylib 0x00007fffb018cd42 __pthread_kill + 10
1 libsystem_pthread.dylib 0x00007fffb027a457 pthread_kill + 90
2 libsystem_c.dylib 0x00007fffb00f2420 abort + 129
3 com.unity3d.UnityEditor5.x 0x0000000100fc3741 HandleSignal(int, __siginfo*, void*) + 81
4 libmono.0.dylib 0x0000000139b4453d mono_chain_signal + 75
5 libmono.0.dylib 0x0000000139a8bb7e mono_sigsegv_signal_handler + 210
6 libsystem_platform.dylib 0x00007fffb026db3a _sigtramp + 26
7 ??? 0x0000000116c2ab20 0 + 4676823840
8 com.unity3d.UnityEditor5.x 0x000000010047cf8f WebViewWindow::SetNativeParentWindow(void*) + 31
9 ??? 0x000000014a05c7aa 0 + 5536860074

Crashes in 2017.3.0b3.
Does not crash in 2017.2.0f2.