Reproduction Steps:
1. Open project
2. In Hierarchy select "Particle System"
3. Under Particle System component set any of the fields to "Random Between Two Constants"
Note: it should work
4. Select "Particle System" child "Derbis"
5. Try step 2
6. Notice that nothing happens (field does not change its type)
7. Disable Particle Simulation "Stop"
8. Try step 2 again
9. Notice that now it works as expected

Expected behaviour: It is always possible to change field type.
Actual result: If Simulation is running changing Sub Emitters field type does nothing

Reproduced on versions: Unity 5.6.0a6, 5.6.0b1
Did not reproduce on versions: Unity 5.4.3p3, 5.5.0p1, 5.6.0a1
Project crashes on versions between 5.6.0a1 and 5.6.0a6