How to reproduce:
1. Open attached "TreeCollidersRepro" project and open "save 3" scene
2. Play the game, crash doesn't occur, exit playmode
3. Inspect the "island" GameObject, go to it's "Trees" settings
4. Check "Enable Tree Colliders" setting and enter playmode

Expected result: Editor doesn't crash
Actual result: Editor crashes

Reproducible with - 5.6.0b5, 5.5.1p1

Notes:
Couldn't reproduce the issue with 5.4.4p2 version, since the project produces a lot of errors there