A famouse error that i have

hello everyone i am new in this forum (and please forgive me for my english).
my problem is this:
i have scrolling thumbnails (one movieClip with many movieClips inside), each thumbnail is a movieClip which is a button, and each one has a class.
the movieClips which are on the stage on the fla file working when i start the swf file , but those who are above or under the stage on the fla file which come on the scrolling, when i press on them there is this error:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at DragDrop10/::drag()

i hope i explaind my problem well and you’ll be able to help me.

this is the class code:
package {
import flash.display.MovieClip;
import flash.events.MouseEvent;
import flash.net.URLRequest;
import flash.net.URLLoader;
import flash.display.Loader;
import flash.events.Event;

public class DragDrop10 extends MovieClip {
public var target:MovieClip;
private var originalX:Number;
private var originalY:Number;

public function DragDrop10() {
originalX=this.x;
originalY=this.y;

this.addEventListener(MouseEvent.MOUSE_DOWN, drag);
}
private function drag(event:MouseEvent):void {
var req:URLRequest= new URLRequest(“Joct.swf”);
var loader:Loader = new Loader();
loader.contentLoaderInfo.addEventListener(Event.COMPLETE, fileLoaded);
loader.load(req);
target.addChild(loader);
loader.x = -540;
loader.y = -400;

}
function fileLoaded(event:Event):void {

}

}
}

and this is the main time line code (include the scrolling code):
stop();
this.addEventListener(Event.ENTER_FRAME, scrolly);
function scrolly(event:Event):void
{
var myVar:Boolean = true;
if(this.mouseX>75)
{
myVar=false;
}
if(this.mouseX<51)
{
myVar=true;
}

if(this.mouseY>550 && myVar==true)
{
all2_mc.prevFrame();
all2_mc.prevFrame();
all2_mc.prevFrame();

}
else
{
all2_mc.play();
}
if(this.mouseY<300 && myVar==true)
{
all2_mc.nextFrame();
all2_mc.nextFrame();
all2_mc.nextFrame();

}
else
{
all2_mc.stop();
}

if(this.mouseY>550 && this.mouseY<300 && myVar==true)
{
all2_mc.stop();
}
}

stage.scaleMode = flash.display.StageScaleMode.NO_SCALE;
stage.align = flash.display.StageAlign.TOP_LEFT;

var buttons:Array = [all2_mc.all_mc.jan_mc, all2_mc.all_mc.feb_mc,
all2_mc.all_mc.march_mc, all2_mc.all_mc.april_mc,all2_mc.all_mc.oct_mc ]
for (var i:uint = 0; i<buttons.length;i++)
{
buttons*.target=stam_mc;
}