Stopped access violation when erasing particles.
Fixed script editor crash when script calls Script.stop(). Also made the
processEraseMessage wait until it can lock to ensure particles are
deleted.
Merge branch 'master' of https://github.com/highfidelity/hifi
Fixed my mistake by unlocking the lock.
Merge pull request #2899 from Barnold1953/master

Particle and script editor crash fixes.