Editor crashes when switching between Metal and OpenGL with a custom script window open

1. Open attached project (blank scene, only has one .cs file)
2. Window -> ProgressbarTest (this will open an new empty window)
3. Edit -> Project Settings -> Player -> Enable/disable "Metal Editor Support" to swap between Metal and OpenGL
4. Click "Apply" for the confirmation popup

Expected: Editor swaps between Metal/OpenGL successfully.
Observed: Editor crashes immediately.

Occurring on: 5.6.0f3 onwards until 2017.2.0b1 (current latest trunk)
Not occurring: 5.5.3f1 and older (Metal editor not supported yet)

Probably related to https://fogbugz.unity3d.com/f/cases/902189/