On Mac, Unity version installs to /Applications/Unity/ by default. Users who want to have multiple versions on their machine usually rename the folder. This crash occurred when I forgot to do that for 2017.3.0b5, and I installed 2018.1.a1 while 2017.3.0b5 was still open. The installation finished, and when I clicked back onto the Editor, it crashed.

Repro steps:
1) Install a version of Unity on Mac (I used 2017.3.0b5 initially, but have repro'd with others) and don't rename the folder
2) Open a project in in that version of Unity
3) Install a different version of Unity via Download Assistant (I used 2018.1.0a1 initially)
4) Uncheck 'launch unity' - do not attempt to launch the new version you just installed
5) Go back to the version of Unity that is already open (eg 2017.3.0b5 if following the above example)
6) Crash occurs

Repros on different projects

Does not occur when going from 2017.2 to 2017.3, but a hang occurs when going from 2017.3 to 2017.2