Stage

The display list is the mechanism that Flash uses to render itself onto the users screen. The display list is rooted in the Stage class and consists of all of its descendants (children, grandchildren, and so forth). DisplayObjects on the display list have a specific rendering lifecycle and event propagation mechanism. Objects which are not on the display list will not be rendered and cannot interact with user events.

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The Stage class is the area in the Flash player that all of the content is rendered into. The Stage is the root of the display list. Other components will be rendered in the normal render loop only if they are children or descendants of the Stage class.

Stage is a DisplayObjectContainer class and supports the normal methods of addChild(child:DisplayObject) and removeChild(child:DisplayObject). Outside of these methods, however, many of the properties of DisplayObject to not apply to the Stage since it has no rendering by itself.

Problem

You have a handler attached to the removedFromStage event on your component and this handler is being called unexpectedly. Sometimes called during component initialization and sometimes during resizing.