Well, I reproduced what you experienced by adding a TLF font to the scene in CS5, which should be the reason for this. But this code should solve your problem:
public function Engine():void
{
if( !this.stage )
this.addEventListener( Event.ADDED_TO_STAGE, init );
else
init();
}
private function init(e:Event = null):void
{
this.removeEventListener( Event.ADDED_TO_STAGE, init );
trace(stage);
}
, . , , , . !:)