Steps:
1. Create a three prefabs (A, B, C)
2. Open A and nest B and C
3. Drag B on top of C (in the way it doesn't change it is actual order, stays in the same place)
==>
Editor crashes
If it doesn't, repeat step 3 a few times

Note: Editor-prev.log has the actual call stack

Take a look into movie for details

Cannot reproduce it in 2018.3.0b1, 2018.3.0b5, 2018.3.0b9, so set as regression.
Since it is hard to repro, may be not a regression