Loading an External SWF Files not working

Question

Hello,
So I have a project I am working on in which I would like to teach how to cash a scratch off card. I have made the scratch and made it so it will "scratch"
Before:                                                                                                                                               After:
Now I have saved this out and am trying to load this into my "holder" however I am getting an error:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at ScratchTicket()
When I use this code:
stop();
import flash.events.MouseEvent;
nextPG_btn.addEventListener(MouseEvent.CLICK, nClick);
lastPG_btn.addEventListener(MouseEvent.CLICK, lClick);
function nClick(event:MouseEvent):void{
    nextFrame();
function lClick(event:MouseEvent):void{
    gotoAndStop(3);
var mcExt:MovieClip;
var ldr:Loader = new Loader();
ldr.contentLoaderInfo.addEventListener(Event.COMPLETE, swfLoaded);
ldr.load(new URLRequest("ScratchTicketOnly.swf"));
function swfLoaded(e:Event):void {
    mcExt = MovieClip(ldr.contentLoaderInfo.content);
    ldr.contentLoaderInfo.removeEventListener(Event.COMPLETE, swfLoaded);
    mcExt.x = 50;
    mcExt.y = 50;
    addChild(mcExt);
What am I doing wrong?
Is it because of my coding in the External SWF?
package {
    import flash.display.Sprite;
    import flash.display.MovieClip;
    import flash.events.MouseEvent;
    import flash.ui.Mouse;
    public class ScratchTicket extends MovieClip{
        var scratcherIsActive:Boolean= false;
        var stuffUnderMask:Sprite = new Sprite();
        // init class
        public function ScratchTicket():void{
            stage.addEventListener(MouseEvent.MOUSE_DOWN, mouseIsDown);
            stage.addEventListener(MouseEvent.MOUSE_MOVE, mouseIsMoving);
            stage.addEventListener(MouseEvent.MOUSE_UP, mouseIsUp);
            // make mask           
            stuffUnderMaskClip.mask = stuffUnderMask;
            addChild(stuffUnderMask);
            makeCursor();
        // helper functions
        private function makeCursor():void{           
            trace("Custom cursor can go here");
        // hover effects
        private function mouseIsDown(e:MouseEvent):void{
            //Mouse.hide();
            Mouse.cursor="hand";
            scratcherIsActive = true;
        private function mouseIsUp(e:MouseEvent):void{
            //Mouse.show();
            Mouse.cursor="arrow";
            scratcherIsActive = false;
        private function mouseIsMoving(e:MouseEvent):void{
            if(scratcherIsActive){
                stuffUnderMask.graphics.beginFill(0x000000);
                stuffUnderMask.graphics.drawEllipse(mouseX, mouseY,60, 60);
                stuffUnderMask.graphics.endFill();
    }// class
}// package
Any help is great!

Answer 1

Make sure you check for errors on loading..
import flash.events.IOErrorEvent;
ldr.contentLoaderInfo.addEventListener(IOErrorEvent.IO_ERROR, uhOhError);
function uhOhError(e:Event):void
     trace(e);
     // probably put some visual text field here to display the error
Aside that, on the first block of code mentioned, is that a frame script or running from somewhere in a class? If in a class and it doesn't extend a display object then you can't addChild(mcExt) without erroring.