I also got this error. I am running PlayMaker 1.2.1. I did, however, manage to track down the cause.
In FsmComponentInspector.cs, the call to FsmVariable.GetFsmVariableList was only passing in one parameter. I fixed this by changing line 224 of FsmComponentInspector.cs from:
fsmVariables = FsmVariable.GetFsmVariableList(fsmComponent.Fsm.Variables);
to:
fsmVariables = FsmVariable.GetFsmVariableList(fsmComponent.Fsm.Variables, fsmComponent.Fsm.Owner);