addedToStage

The addedToStage event fires when a component becomes part of the display list. The display list is rooted in the Stage component and contains all of its descendants. When your component is on the Stage it participates in the standard render loop and the event dispatching loop.

Keep in mind that even though your component is not in the display list, it can still be asked to render itself using the draw method of BitmapData.

Problem

You have a PopUpButton that is failing with a null error unexpectedly.

Solution

Reinitialize the popUp property of the PopUpButton every time it receives the addedToStage event. You can save a reference to the class in your parent component.