Are you missing a section of script?
I know in the default scripts, that method is in the Game_Party class. I'm not sure if it was moved for BABS, but you can try adding changing the line so that it reads:
return $game_party.item_number(id) != 0
If someone else can say for sure if this method was moved for BABS, it would be most helpful. Otherwise I am not so sure of the wisdom in changing it where the method is being called from a class other than what it should be, which in this example appears to be in Game_Player.