Despartely need help with NULL ERROR

programming flash
1

I have One SWF loading Two External SWFs seprately that have dataGrid components in them. The Externals load and run fine on their own but when I try to load them into CORE SWF I get …

TypeError: Error #2007: Parameter child must be non-null.
at flash.display::DisplayObjectContainer/addChildAt()
at fl.controls::BaseButton/drawBackground()
at fl.controls::LabelButton/draw()
at fl.core::UIComponent/drawNow()
at fl.controls::DataGrid/drawList()
at fl.controls::DataGrid/draw()
at fl.core::UIComponent/callLaterDispatcher()

Here is the AS3 Code for the CORE SWF:

//* MAKE BUTTONS STAY ACTIVE WHEN CLICKED *//
var aClip:MovieClip = this.getChildByName("a_mc") as MovieClip;
var bClip:MovieClip = this.getChildByName("b_mc") as MovieClip;
var hash:Dictionary = new Dictionary();
aClip.addEventListener(MouseEvent.CLICK, handleClick);
bClip.addEventListener(MouseEvent.CLICK, handleClick);
/*aClip.addEventListener(MouseEvent.MOUSE_OVER, handleOver);
bClip.addEventListener(MouseEvent.MOUSE_OVER, handleOver);*/
var selected:MovieClip = aClip; 
selected.gotoAndStop(10); 
trace("selected");
function handleClick(e:MouseEvent):void 
{     
 var target:MovieClip = e.currentTarget as MovieClip;     
 if (target != selected) 
 {         
  selected.gotoAndStop(1);         
  selected = target;         
  selected.gotoAndStop(5);     
  } 
}
//* ALL ABOUT LOADERS *//
var swfholder:Loader=new Loader();
a_mc.addEventListener(MouseEvent.CLICK, loadLeg);
b_mc.addEventListener(MouseEvent.CLICK, loadHouse);
swfholder.load(new URLRequest("04_23_pensions_pe.swf")); //starts movie with Pensions loaded
swfholder.contentLoaderInfo.addEventListener(Event.COMPLETE, onSwfLoaded)

function onSwfLoaded(event:Event):void  //prepares Retirees loader, removes Pensions 
{
   this.addChild(swfholder);
   swfholder.x =5; 
   swfholder.y =125; 
   swfholder.width =885;
   swfholder.height =430;
   b_mc.addEventListener(MouseEvent.CLICK, loadHouse);
   a_mc.removeEventListener(MouseEvent.CLICK, unloadthis);
}
function loadLeg(event:MouseEvent):void
{
 this.addChild(swfholder);
 swfholder.load(new URLRequest("04_23_pensions_pe.swf")); //loads Legislature swf
 swfholder.x =0; 
 swfholder.y =0;
 swfholder.width =880;
    swfholder.height =308;
 b_mc.addEventListener(MouseEvent.CLICK, unloadthis);
 a_mc.removeEventListener(MouseEvent.CLICK, loadLeg);
}
function loadHouse(event:MouseEvent):void
{ 
 this.addChild(swfholder);
 swfholder.load(new URLRequest("04_23_pensions_re.swf")); //loads House swf
 swfholder.x =0; 
 swfholder.y =0; 
 swfholder.width =880;
    swfholder.height =308;
 b_mc.removeEventListener(MouseEvent.CLICK, loadHouse);
 a_mc.addEventListener(MouseEvent.CLICK, unloadthis);
}
function unloadthis(event:Event):void
{
 removeChild(swfholder);
 a_mc.addEventListener(MouseEvent.CLICK, loadLeg);
 b_mc.addEventListener(MouseEvent.CLICK, loadHouse);
}