Won't let me reset child index

Question

I've got multiple images being loaded from an xml document and when they're created they add it to the stage and call a setIndex function to set it to 0. But when I try to the same object later on when they're called I get this compiler error:
I marked in bold the lines that relate. Out of the last two lines I marked bold I get this compiler error when I use the first one:
     1118: Implicit coercion of a value with static type Object to a possibly unrelated type flash.display:Sprite.
and this output error when from the last bold line:
    TypeError: Error #2007: Parameter child must be non-null.
function xmlLoaded(e:Event):void {
     e.currentTarget.removeEventListener("complete",xmlLoaded);
     myXML = new XML(e.target.data);
     xDoc = new XMLDocument();
     xDoc.ignoreWhite = true;
     xDoc.parseXML(myXML.toXMLString());
     thumbSrc = new Vector.<String>(xDoc.firstChild.childNodes.length);
     imgSrc = new Vector.<String>(xDoc.firstChild.childNodes.length);
     imgArray.push(thumbSrc,imgSrc,spriteSrc);
     //trace(xDoc.firstChild.childNodes[0].firstChild.firstChild);
     for (var i:int = 0; i < xDoc.firstChild.childNodes.length; i++) {
          var thumbTainer:Sprite = new Sprite();
          thumbTainer.name = "thumbHolder" + i;
          spriteSrc.push(thumbTainer);
          thumbSrc[i] = xDoc.firstChild.childNodes[i].firstChild.nextSibling.firstChild;
          imgSrc[i] = xDoc.firstChild.childNodes[i].firstChild.firstChild;
          if (i == 0) {
               loadThumb(i);
function loadThumb(num:int):void {
     if (getter.vector[2][num] != null) {
         stage.addChild(getter.vector[2][num]);
          setIndex(getter.vector[2][num]);
          var thumbLoader:Loader = new Loader();
          thumbLoader.load(new URLRequest(getter.vector[0][num]));
          thumbLoader.contentLoaderInfo.addEventListener("progressEvent",imgProgress,false,0,true);
          thumbLoader.contentLoaderInfo.addEventListener("complete",function(e:Event){thumbLoaded(t humbLoader,num); },false,0,true);
function setIndex(holder:Sprite) {
     stage.setChildIndex(holder,0);
function thumbLoaded(loader:Loader, num:int):void {
     var thumbBit:Bitmap = Bitmap(loader.content);
     var thumbData:BitmapData = BitmapData(thumbBit.bitmapData);
     var Width:Number = thumbData.width;
     var Height:Number = thumbData.height;
     var holder:Sprite = getter.vector[2][num];
     var rect:Rectangle = new Rectangle(0,0,Width,Height);
     var numX:Number = getNumX(num) * 325;
     var numY:Number = Math.floor(num/3) * 350;
     thumbData.pixelDissolve(thumbData,rect,new Point(400,400),2);
     holder.addChild(thumbBit);
     holder.alpha /= 2;
     holder.addEventListener("rollOver", alphaIn);
     holder.addEventListener("click",showImage);
     holder.x = stage.globalToLocal(new Point(0,0)).x - 350 + numX;
     holder.y = stage.globalToLocal(new Point(0,0)).y - 350 + numY;
     holder.z = 900;
     holder.buttonMode = true;
     loadThumb(num + 1);
     loader.removeEventListener("complete",thumbLoaded);
function showImage(e:MouseEvent):void {
     var others:Object;
     var obj:DisplayObject;
     var midStageX:Number = stage.stageWidth/2 - e.currentTarget.width/2 - 120;
     var midStageY:Number = stage.stageHeight/2 - e.currentTarget.height/2 - 155;
     for (var i:int = 0;i < getter.vector[0].length;i++) {
          if (e.currentTarget.name != "thumbHolder" + i) {
               others = stage.getChildByName("thumbHolder" + i);
               setIndex(others);
               //TweenLite.to(others,count,{alpha:0});
          } else {
               //obj = e.currentTarget as DisplayObject;
               stage.setChildIndex(obj,numChildren-1);
     TweenLite.to(e.currentTarget,count,{x:midStageX,y:midStageY});
     e.currentTarget.alpha = 1;
     e.currentTarget.removeEventListener("rollOut",alphaOut);
     e.currentTarget.addEventListener("enterFrame",zoomIn);
     e.currentTarget.removeEventListener("click",showImage);

Answer 1

1.  if getter.vector[2][num] is a sprite use:
setIndex(Sprite(getter.vector[2][num]));
2.  obj is null.  define it.