Particle system throws errors when using sub emitters and "prewarm" is checked.

Repro steps:
1. Open Mixed scene from attached project.
2. Press play.
3. Observe errors in inspector.

Expected outcome: no errors.

Notes:
- occurs only if prewarm has been checked and the particle system has at least 1 sub emitter (i.e. birth).
- scene Mixed_bug had only 1 particle system and error occurs sometimes (typically first time it's played, then on and off). The more particles, the more likely the error to occur.
- not a regression, happens in 5.3.2f1