Reproduction steps:

1. Open "MinimalRepro.zip"
2. Open "SoldierFreeScene" Scene
3. "GameObject" -> "3D Object" -> "Ragdoll..."
4. Set the appropriate Bones
5. "Create" or Exit

Expected Result: A Ragdoll is created
Actual Result: The Editor crashes

Fixed in: Version 2018.1.0a5

Reproduced with: 2017.3.0a5, 2017.3.0a3, 2017.2.0b6, 2017.1.0p3, 5.6.3f1, 5.6.0b1, 5.6.0a5
Regression introduced in: 5.6.0a5
Did not reproduce on: 5.5.4p2

Note:
Only Reproduces on Mac (Tested on Mac OS 10.12.6)
Changing the "Name" of one of the Bones and the creating a Ragdoll, increases the change of crashing