Controlling loaded swf files help

programming flash
1

Hello, I’m trying to create a universal fla file (shell.fla) in which I can load any swf file and control it from buttons in the shell itself. The buttons being play, pause, rewind, magnify etc. I’ve done all this in AS2 and now I have to do it in AS3. See my website http://www.eudv.de/Pages/flash.html - E-Training - Motor to get a better idea of what I mean.
The shell file has three buttons so far with instances called playMovie, stopMovie changeMovie and a dynamic text field - movieNumber. It is bound to the class Shell. The swf files to be loaded are numerical 1.swf, 2.swf, 3.swf etc. If I can get playMovie, stopMovie to work I can then do the rest.
Here is my code so far for the class Shell.as:

package {
import flash.display.MovieClip;
import flash.events.MouseEvent;
import flash.events.Event;
import flash.display.Loader;
import flash.net.URLRequest;

public class Shell extends MovieClip {

    var movie:MovieClip;
    var N:uint;

    public function Shell() {
        N=0;
        // Buttons to control the movieClip instance movie
        playMovie.addEventListener(MouseEvent.CLICK, onPlayClick);
        stopMovie.addEventListener(MouseEvent.CLICK, onStopClick);
        //Button to change movieClip
        changeMovie.addEventListener(MouseEvent.CLICK, onChangeClick);
        //maybe not necessary?
        addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }
    private function loadHandler(event:Event) {
        var movie:MovieClip=event.target.content;
        addChild(movie);
        movie.stop();
    }
    private function onChangeClick(event:MouseEvent):void {
        N++;
        //Dynamic text to show which swf file is running
        movieNumber.text=N+".swf";
        if (N>0) {
            //loads the next swf file
            var nextMovie:URLRequest=new URLRequest(N+".swf");
            var swfLoader:Loader = new Loader();
           swfLoader.contentLoaderInfo.addEventListener(Event.COMPLETE, loadHandler);
            swfLoader.load(nextMovie);
        }
    }
    public function onPlayClick(event:MouseEvent):void {
        movie.play();
    }
    public function onStopClick(event:MouseEvent):void {
        movie.stop();
    }
    public function onEnterFrame(event:Event):void {
    }
}

}
The problem:

  1. How can I kill, unload, removeChild the preceding swf file which has been loaded? When I load a swf file it creates a new loader.
  2. On the onPlayClick and onStopClick buttons I get the following error message:
    TypeError: Error #1009: Der Zugriff auf eine Eigenschaft oder eine Methode eines null-Objektverweises ist nicht möglich.at Shell/onPlayClick()

Has anybody got any ideas please?