Unity will crash when a user attempts to undo an "apply" action to an upgraded disconnected prefab.

To reproduce:
1) Download + open attached project
2) Select the disconnected prefab in the hierarchy view
3) Click on overrides + "Apply all"
4) Press Ctrl+Z / Cmd+Z to undo

Unity should proceed to crash.

How to produce this situation:
1) Open Unity 2018.2 or lower
2) Create a prefab and an instance of the prefab in the scene
3) Select the instance of the prefab in the hierarchy + "break prefab instance" by going GameObject -> Break Prefab Instance
4) Open the project in 2018.3 or later
5) Repeat above steps!

Regresses 2018.3.0a9 (seems to be the first version that prefab upgrades are working)