Two fixes: 

* ParticleSystem should also override `onExit` to unschedule update since in `onEnter`, it will be scheduled again. Otherwise, it'll trigger an assert.
* User object has to be released before others, since userObject may have a weak reference of this node.
It may invoke `node->stopAllAction();` while `_actionManager` is null if the next line is after `CC_SAFE_RELEASE_NULL(_actionManager)`.