Null object Error Help Please ..........

package {
import flash.events.MouseEvent;
import flash.events.Event;
import flash.utils.Timer;
import flash.events.TimerEvent;
import flash.utils.setTimeout;
import flash.net.URLRequest;
import flash.display.MovieClip;
import flash.display.Loader;
import fl.transitions.*;
import fl.transitions.easing.*;
public class gallery {
private var ref:Object;
private var lastObj:Object;
private var firstObj:Object;
private var numberOfImages:Number=9;
private var arr:Array=[];
private var Width:Number;
private var Height:Number;
private var imageCount:Number=1;
private var lastValue:Number;

private var timer:Timer;
private var firstObject:Object;
private var lastObject:Object;
public function gallery(obj:Object) {
ref=obj;
ref.image_mc.oldX=ref.image_mc.x;

ref.nxt_mc.addEventListener(MouseEvent.CLICK,moveNext);
ref.prev_mc.addEventListener(MouseEvent.CLICK,movePrev);
loadAllImage();
for (var i=1; i<=numberOfImages; i++) { attachClip(i,lastObj); arr.push(i); } firstObject=ref.image_mc.getChildAt(0); lastObject=ref.image_mc.getChildAt(ref.image_mc.numChildren-1); } private function movePrev(evnt:MouseEvent) { var currentX=ref.image_mc.oldX+(Width+Number(10)); var myTweenX = new Tween(ref.image_mc, "x", Strong.easeOut,ref.image_mc.x,currentX,0.5, true); var myTweenY = new Tween(ref.image_mc, "y", Strong.easeOut,ref.image_mc.y,ref.image_mc.y,0.5, true); ref.image_mc.oldX=currentX var id=setTimeout(removeLast,200) } private function moveNext(evnt:MouseEvent) { var currentX=ref.image_mc.oldX-(Width+Number(10)); var myTweenX = new Tween(ref.image_mc, "x", Strong.easeOut,ref.image_mc.x,currentX,0.5, true); var myTweenY = new Tween(ref.image_mc, "y", Strong.easeOut,ref.image_mc.y,ref.image_mc.y,0.5, true); ref.image_mc.oldX=currentX var id=setTimeout(removeFirst,200) } private function removeFirst() { try { if (lastObject==null) { lastObject=ref.image_mc.getChildByName('thumNail_'+numberOfImages); } var firstId:Number=firstObject.name.split('_')[1]; var lastId:Number=lastObject.name.split('_')[1]; firstObject.x=(lastObject.x+lastObject.width)+Number(10); if ((firstId+1)>numberOfImages) {
firstId=0;
}
firstObject=ref.image_mc.getChildByName('thumNail_'+(firstId+1));
lastObject=ref.image_mc.getChildByName('thumNail_'+firstId);
} catch (Err:*) {
}
}

private function removeLast() {
try {
if (firstObject==null) {
firstObject=ref.image_mc.getChildByName('thumNail_'+1);
}
if (lastObject==null) {
lastObject=ref.image_mc.getChildByName('thumNail_'+numberOfImages);
}
var firstId:Number=firstObject.name.split('_')[1];
var lastId:Number=lastObject.name.split('_')[1];
lastObject.x=(firstObject.x-firstObject.width)-Number(10);

if ((lastId-1)<0) {
lastId=numberOfImages
}
lastObject=ref.image_mc.getChildByName('thumNail_'+(lastId-1));
firstObject=ref.image_mc.getChildByName('thumNail_'+lastId);


} catch (Err:*) {
}
}
private function loadAllImage() {
try {
var url:String='images/image_'+imageCount+'.jpg';
imageCount++;
var req:URLRequest=new URLRequest(url);
var loader:Loader=new Loader();
loader.load(req);
loader.contentLoaderInfo.addEventListener(Event.COMPLETE,loadingCompleted);
} catch (Err:*) {
}
}
private function loadingCompleted(evnt:Event) {
loadAllImage();

}

private function attachClip(arg:Number,Obj:Object,num:Number=0) {
var thumNail:thumnail_mc=new thumnail_mc();
thumNail.name='thumNail_'+arg;
Width=thumNail.width;
Height=thumNail.height;
if (Obj) {
if (num!=0) {
thumNail.x=(Obj.x-Obj.width)-Number(10);
firstObj=thumNail;
} else {
thumNail.x=(Obj.x+Obj.width)+Number(10);
}
} else {
}
lastObj=thumNail;
thumNail._txt.text=String(arg);
ref.image_mc.addChild(thumNail);
loadThumNailImages(thumNail,arg);

}
private function loadThumNailImages(thumNail:Object,arg:Number) {
var url:String='images/image_'+arg+'.jpg';
var req:URLRequest=new URLRequest(url);
var loader:Loader=new Loader();
loader.load(req);
loader.contentLoaderInfo.addEventListener(Event.COMPLETE,imageLoadCompleted);
thumNail.image.addChild(loader);
}
private function imageLoadCompleted(evnt:Event) {
var target=(evnt.target.content);
target.parent.parent.width=Width;
target.parent.parent.height=Height/2;
}
}
}