Repro steps:
1. Open Main scene from attached project.
2. Select the Debris sub emitter (child of Particle System)
3. In the inspector, open the Debris Sub Emitter module and add a new sub emitter
4. Using the circle in the inspector, assign Debris to itself.
5. Unity crashes

Expected outcome: Unity to display an error message that this would be a circular reference.

Notes:
- Happens in 5.5.0f1, 5.5.0b11
- Only happens when assigning directly from the inspector; dragging the particle system displays the appropriate warning.
- Only happens on sub emitters. If the parent particle system is assigned as a circular reference the appropriate warning will be shown.
- Regression from 5.4.2p2 (new sub emitter module was added in 5.5 which most likely introduced the issue)