You're probably calling "self.stage" in one of your init-methods. In classes other than the stage, the property "self.stage" returns nil in the init-method. That's because the method just calls "self.parent" until it finds a parent that is the stage. In the init-method, however, your object does not have a parent yet, thus you cannot access the stage.
You can either make a separate method to start the animations (called after the object is added to a parent) or you start the animations in an added-to-stage event listener. The movie scene in the Demo app does it like this, you can have a look at this class to see how to do it.