Steps to reproduce:

1. Open new project
2. Switch to iOS
3. Go to 'Player Settings' and change the following options:
- Scripting Backend: Mono2x
- Stripping Level: 'Strip Byte Code' or 'Use micro mscorlib'
4. Trigger a build
5. Before 'AOT' part in the building progress bar shows up, drag some other window on top of Unity's progress bar and focus it so that the progress bar becomes hidden
6. Wait a couple seconds, Unity Editor will crash

Notes:
This might be happening on some different setups as well (besides Mono2x scripting backend and 'Strip Byte Code' or 'Use micro mscorlib' stripping levels), but I was only able to reproduce this issue on these conditions.
Building for Android with the same scripting backend and stripping levels doesn't crash the editor.

Reproduced with: 5.4.3p4, 5.5.0p2
Not reproducible with: 5.4.3p1, 5.5.0p1