need help non-null error: xml or onComplete?

Thats is my question, I just have this code on the frame  that I want it to play on and it plays on the first try, after that it's not loading.

I don't know what i am doing wrong or how or where to code to put the listener.  I've tried so many things, obviously not the right thing!

var ss1Req:URLRequest = new URLRequest("slideshow/slideshow.swf");
var ss1loader:Loader = new Loader();

        _1a.x = 10;
        _1a.y = 70;
        ss1loader.load(ss1Req);
        _1a.addChild(ss1loader);
       
   
    ss1loader.unload();

This is the buttoncode that takes user to the page

smm1_btn.addEventListener(MouseEvent.MOUSE_DOWN, slide);
        function slide(event:MouseEvent):void  {
            if (this.vidPlayer == !null)
        this.vidPlayer.stop();
        SoundMixer.stopAll();
            MovieClip(this.parent).gotoAndStop("photo");
           
        }

This is the site :

http://www.stacykessler.com/test.html

Thanks,

Show code of how you add listener.