To reproduce:
1. Open attached project;
2. Open "Behavior" scene;
3. Click on "Particle System" in hierarchy;
4. Click on "sub emitter" in hierarchy (Particle System > sub emitter);
5. Repeat 3rd and 4th steps until Editor freezes.

Expected result: Editor doesn't freeze.
Actual result: Editor freezes.

Note: this doesn't happen when "Send Collision Messages" (Particle System > Collision > Send Collision Messages) is unmarked on both Particle System and its Sub Emitter.

Reproduced on versions: 5.4.4p4, 5.5.0f3, 5.5.2f1, 5.6.0a5.
Not reproduced on versions: 5.6.0a6, 5.6.0b10, 2017.1.0a2.