Referencing a movieclip instance from an (loaded) external swf.

Question

Hi everybody
I'm new to AS3. Even experienced in AS2, i'm a bit confused
in getting the following result.
Let's assume the following.
The mainmovie has a movieclip instance called 'window1'. I
need to have loaded a movieclip (submov.swf) in the mainmovie. As a
final result a button is to be added to 'window1' after everytime
an instance of submov is created.
The document class of submov.swf is as follows
package
import flash.display.*;
import fl.controls.Button;
public class submov extends Sprite
function submov():void
var firstbutton:Button = new Button();
firstbutton.move(0,0);
firstbutton.setSize(70,30);
firstbutton.label = "Submit";
stage.window1.addChild(firstbutton); //this is the place
where error occurs (my assumption)
the first frame of the mainmovie express the following code
to load mainmov.
import flash.display.*;
import flash.net.URLRequest;
var ldr:Loader = new Loader();
var url:String = "mainmov.swf";
var urlReq:URLRequest = new URLRequest(url);
ldr.load(urlReq); // the first instance of mainmov created
here
var a:submov = new submov(); // the second instance of
mainmov created here
var b:submov = new submov(); // the third one created here
so the final result must be the button symbol created three
times in 'window1' (the position of the buttons may be ignored).
But what i'm getting is an error reporting
TypeError: Error #1009: Cannot access a property or method of
a null object reference.
at submov$iinit()
PLEASE HELP ME at this stage to overcome the problem.

Answer 1

no,
i tried that too, but there is the following error reported.
TypeError: Error #1009: Cannot access a property or method of
a null object reference.
at submov$iinit()