Jobu's Context Menu

I have implemented one of these, and although it seems to work correctly, it fails when I select the menu ite, giving the error:
NSReceiverEvaluationScriptError: 3 (1)

I have set up handlers for the “choose menu item” for each item in the menu, but even with nothing in the handler I get the error.

The really odd thing is that very occasionally it works! But, the next time I build the project it stops working agan!?