I have this error but I can't find it anywhere in the code. Do you know what it is?

Error running script: Error compiling expression '(room.parent.grid_width - room.grid_width) /2.0': ArithmeticElement: Operation 'Subtract' is not defined for types 'Object' and 'Int32'

I would guess room.parent.grid_width=null for some reason. Check what the room's parent is, and see if it has a value set for that attribute.