Loading an External SWF Files not working

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!