Steps to reproduce:
1. Open user attached project ("softbodyCloth")
2. Create a plane and a cube
3. Add a rigidbody and a cloth component to the the cube
4. Edit constraints and change max distance and penetration to zero
5. Run the scene
6. Change the bending stiffness value

Expected result: editor should not crash when the bending stiffness is changed
Actual result: editor crashes when the bending stiffness is changed

Reproduced in: 2017.1.0b3, 5.6.0p4, 5.6.0f3, 5.6.0b4, 5.6.0b3
Not reproduced in: 5.6.0b2, 5.6.0a1, 5.5.3p2
Regression since: 5.6.0b3