not my day today, I needed help earlier today, Sorry but again I have to plea for help.

I'm trying to unload and repopulate my xml menu with another xml menu and I'm failing. Any help will be greatly appreciated. Sincerely Xiaz

Code:
var a = "menu.xml"
var b = "menu2.xml"

btn1.addEventListener(MouseEvent.CLICK,btn_click);
btn2.addEventListener(MouseEvent.CLICK,btn_click);

/////my function to unload and repopulate the menu/////
function btn_click(event:MouseEvent){
   var selected_list:String
   if(event.target == btn1){
      selected_list = a;
      menuHolder.removeAll();
   }else if(event.target == btn2){
      selected_list = b;
      menuHolder.removeAll();
   }
   loader.load(new URLRequest(selected_list));
}

/////loads my xml /////
var xml:XML;
var loader.load(new URLLoader();

loader.load(new URLRequest("menu1.xml"));
loader.addEventListener(Event.COMPLETE,xmlLoaded);

function xmlLoaded(e:Event):void{
   if ((e.target as URLLoader) !=null){
      xml = new XML(loader.data);
      createMenu();
   }
}

/////holder that contains items/////
var menuHolder:MovieClip = new MovieClip();
I get this error everytime I click on either of the two buttons
Code:
TypeError: Error #2007: Parameter url must be non-null.
at flash.net:URLStream/load()
at flash.net::URLLoader/load()
at Vert_fla::MainTimeline/btn_click()