Scripted attribute plugins look for PRC in the wrong spot

I'm 99% sure the Scripted plugin has the wrong default Function for locating the PRC. It's walking two levels up from the ARC to the RPC and then to the PRC, but we dropped the RPC out of the middle of that.

Kind of wish we'd made getParent() non-null and threw on it, would have been caught that way.

Probably none of the unit tests actually check to see if the PRC is non-null.