xml as3 problem

Getting this error when I try to test the xml as3 script:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at xml_images_fla::MainTimeline/processXML()
    at flash.events::EventDispatcher/dispatchEventFunction()
    at flash.events::EventDispatcher/dispatchEvent()
    at flash.net::URLLoader/onComplete()

Can anyone see if the code is so far not working properly or is it somewhere esle?

import flash.net.URLLoader;
import flash.net.URLRequest;
import flash.display.MovieClip;
import flash.events.Event;
import flash.display.Loader;

var columns:Number;

var my_x:Number;
var my_y:Number;
var my_thumb_width:Number;
var my_thumb_height:Number;
var my_images:XMLList;
var my_total:Number;
var container_mc:MovieClip;

var XMLLoader:URLLoader = new URLLoader();

XMLLoader.load(new URLRequest("gallery.xml"));

XMLLoader.addEventListener(Event.COMPLETE, processXML);


function processXML(e:Event):void{

    var myXML:XML = new XML(e.target.data);
    columns = myXML.@COLUMNS;
    my_x = myXML.@XPOSITION;
    my_y = myXML.@YPOSITION;
    my_thumb_width = myXML.@WIDTH;
    my_thumb_height = myXML.@HEIGHT;
    my_images = my_images.length();

    createContainer();
    callThumbs();

}

function createContainer():void{

    container_mc = new MovieClip();
    container_mc.x = my_x;
    container_mc.y = my_y;
    addChild(container_mc);

}

function callThumbs():void{

    for (var i:Number = 0; i < my_total; i++){
        var thumb_url = my_images[i].@THUMB;;
        var thumb_loader = new Loader();
        thumb_loader.load(new URLRequest(thumb_url));
        thumb_loader.contentLoaderInfo.addEventListener(Event.COMPLETE, thumbLoaded);

    }
}


function thumbLoaded(e:Event):void{

    var my_thumb:Loader = Loader(e.target.loader);
    container_mc.addChild(my_thumb);
}