In the playfield options menu, you may choose the 'save as SqueakPage at
url...' option.  If you do and decide to cancel, a new SqueakPage
instance is sent the url: message with an UndefinedObject as argument.
The method tries valiantly to be bulletproof by testing the argument to
url: to see if it isEmpty.  Unfortunately, UndefinedObject implements
isNil and isEmptyOrNil, but not isEmpty.

The fix would be to either add isEmpty to UndefinedObject, or else to
change the url: method to use isEmptyOrNil.  I would guess that the
latter is intended.

--
Matt Rosen
San Joaquin Delta College