add trace statements to confirm which object is null.
also do you remove these swfs from the displaylist before you deconstruct them?
if so I think the stage property gets set to null (since they are not on the stage anymore)
and so stage.removeEventListener won't work
ya i am running deconstruct function before remove those swf. when i trace stage in child swf's then it showing null.
if it's stage that's null then the object can't be in the DisplayList
an alternative that by passes your problem but does not solve it
when adding your stage.removeEventListener(Event.RESIZE, sizeListener);
you could use
stage.removeEventListener(Event.RESIZE, sizeListener, false, 0, true);
this means the listener is eligible for garbage collection and you don't need to remove the listener