Display vpaid ad in custom player using action script 3

1

I have developed custom player using action script 3 to play vpaid ad. I have XML tags for eg :“http://ad2.liver.com/?LR_PUBLISHER_ID=342431&LR_SCHEMA=vast2-vpaid&LR_AUTOPLAY=1&LR_CONTENT=6&LR_TITLE=@videoTitle&LR_VIDEO_ID=@videoId” which returns vpaid ad(.swf file) which is suppose to be displayed in the player but its giving some error as “TypeError: Error #2007: Parameter child must be non-null.at flash.display:: DisplayObjectContainer/addChild()”. Why it is giving this error?