I learned today that the ADDED_TO_STAGE event constant was not added until flash player 9.0.28.0. This can typically cause the error TypeError: Error #2007: Parameter type must be non-null. at flash.events::EventDispatcher/addEventListener() for people with flash player 9 prior to 9.0.28, for instance flash player 9.0.16. (Yes, I have added the versions to help google find this post.)
I hope this post can help someone find the problem :-)
