Hi.
This is probably not a bug
There is a 'Set property' action that has an object missing.
or there is no object set in the action, or it gets removed somehow while playing.
Do you have errors in the PlayMaker window (bottom left)?
If so click on it to open the 'error check' then click on the error, it will bring you to the fsm/state with the error.
If not does this happen only when you play the game?
if so you need to try to find when and where.