ActionScript 3.0 :: Display Object Must Be A Child Of The Caller?

This is the error I am getting....This is the situation I am using addChild and removeChild. I have two main movieclips that this is based around. workss_mc and work_mc. workss_mc is the slideshow, work_mc is the selection grid. When you click on an object in work_mc I am adding adding a child (workss_mc) and removing work_mc at the same time. That works fine; however, in workss_mc I have a back button, this back button is used to remove works_mc once it is removed it sends you back to work_mc. Once you are back on work_mc everything should work completely the same, however, it's like it's ignoring the code.Here is the code for the button:

Code:
urbanthmb_mc.addEventListener(MouseEvent.CLICK, clickHandler);
function clickHandler(event:Event) {
var workss_mc = new mc_workss();[code].......