I have one SWF loading two external SWFs (CORE loads PE and RE swfs). Separately the PE and RE run pefectly fine. However when I try to load into the CORE SWF I get:

Actionscript Code:
TypeError: Error #2007: Parameter child must be non-null.

I have been looking at this for three days and cannot see where I am going wrong. Please Help

Here is the CORE Code:

Code:
//* 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);

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);
}