Actionscript 3 Unload error

[rhx2060]

I have a scene ("mother") with several buttons. Every button load an external swf ("son01,son02..").
The scene "son.." has buttons that control a collada object(papervision3d) with a function(ERonEnter).
I have a problem: When I press a second button to unload the scene son.. there is an error
Could you help me?, thanks and sorry for my english.

TypeError: Error #1009: No se puede acceder a una propiedad o a un método de una referencia a un objeto nulo.
    at LC15_TEST_fla::MainTimeline/EROnEnter()

and the code:

stop();

import flash.display.Loader
import flash.events.*
import flash.net.URLRequest

var urlRequestLC15:URLRequest=new URLRequest("LC15_TEST.swf")
var loaderLC15:Loader=new Loader()
loaderLC15.contentLoaderInfo.addEventListener(Event.COMPLETE, onComplete)

loaderLC15.x=-5
function onComplete (e:Event):void
{
    addChild(loaderLC15)
}
LC15_mc.buttonMode=true
LC42S_mc.buttonMode=true
LC7_mc.buttonMode=true
LC15_mc.addEventListener(MouseEvent.CLICK,LC15)

function LC15(e:MouseEvent):void
{
loaderLC15.load(urlRequestLC15)
}
close_btn.addEventListener(MouseEvent.CLICK,onClosed)

function onClosed (e:MouseEvent):void{
    loaderLC15.unload()
    removeChild(loaderLC15)
}

and the code function for the scene son

function EROnEnter(e:Event):void {
    this.mScene.renderCamera( mCamera )
    this.stage.quality = "HIGH";
    this.stage.scaleMode = "noScale";
    this.stage.addEventListener(MouseEvent.MOUSE_DOWN, ERMouseDownHandler);
    this.stage.addEventListener(MouseEvent.MOUSE_UP, ERMouseUpHandler);
    this.stage.addEventListener(MouseEvent.MOUSE_WHEEL, ERMouseWheelHandler);
}