Critical Error: Null Object Reference (v3.0)

Hi there, I've recently switched so Stencyl 3.0 and well, my game just doesn't work any more.  I keep getting this error message in the log, [LOG] Critical Error: Null Object Reference, it does this when an object is killed and an actor explosion is created, basically the 'Explode on death' behaviour.  Anyone know why it might be saying this when it was fine in v2.2?  thanks for your help!