Adding "loading" movieclip to dynamic image gallery?

Question

Hello again,
I've completed my XML-driven image gallery, however upon live testing I'd realized I made a rookie mistake and not put some kind of placeholder "loading" movie clip loop while the thumbnails—and subsequent full-size images—are being loaded. The images will randomly appear when loaded and I would like to be able to insert a placeholder movie clip while they load, and swap them up with the thumbnails/images after it has been loaded. The placeholder movie clip is in my library (loadingLoop), but I'm having problems getting it to work correctly.
Below is the original function that processes the XML and adds the thumbnail images; this is where I'm trying to insert the placeholder movie clip per XML child node (every attempt I've made just keeps throwing more errors, so I've omitted my attempts to avoid confusion), and I've attached the XML file for testing. And insight or help would be appreciated.
//Locate the external XML file and trigger xmlLoaded when complete
xmlLoader.load(new URLRequest("data/artGallery.xml"));
xmlLoader.addEventListener(Event.COMPLETE, xmlLoaded);
//Load the XML and process the image locations
function xmlLoaded(event:Event):void {
    xml = XML(event.target.data);
    //parse the nodes in the XML file
    xmlList = xml.children();
    //count the number of nodes in the XML file via XMLList
    trace(xmlList.length());
    //loop to load all of the thumbnails, based on the number of nodes in XMLList
    //"i" = every child node in the XML file
    for (var i:int = 0; i < xmlList.length(); i++) {
        //Add loadingLoop movie clip per node and wait for thumnail to load before swapping out with imageLoader thumbnail images <-- help!!
        //for every node, create a new instance to be put on stage
        imageLoader = new Loader();
        //load each thumbnail from its location per XML node's "thumb" attribute
        imageLoader.load(new URLRequest(xmlList[i].attribute("thumb")));
        //position of thumbnail instances per XML node (horizontally across stage)
        imageLoader.x = i * 110 + 10;//thumbs are 100 x 100 so, +10 to the width, plus +10 from left edge
        //imageLoader.y = 10;
        //for each node in XML list, name the instance with the path to the location of the full size image
        imageLoader.name = xmlList[i].attribute("source");
        //for each node in the XML list, add a drop shadow
        imageLoader.filters = [thumbDShadow];
        //add thumbnails to stage
        addChild(imageLoader);
        dropTween = new Tween(imageLoader, "y", Bounce.easeOut, -100, 10, 1, true);
        //set up thumbnails to wait for a click to execute showPicture
        imageLoader.addEventListener(MouseEvent.CLICK, showPicture);
        //set up thumbnails to scale when rolled over
        imageLoader.addEventListener(MouseEvent.ROLL_OVER, sizeUP);

Answer 1

Somehow I had a feeling this wouldn't be a copy-and-paste job for the full-size images. Sorry to be a bother. This is the showPicture function that is fired off when the thumbnail image is clicked (from our previous example). Two errors are thrown:
When the thumnail is clicked:
"Error #2007: Parameter child must be non-null.
    at flash.display::DisplayObjectContainer/removeChild()
    at MethodInfo-51()"
But the full-size image loads nonetheless. The second loading loop (loadingLoop2) is not removed. Below is the showPicture function:
//Load the full-size images and place onto the stage
function showPicture(event:MouseEvent):void {
    //add new movie clip container
    var mc2:MovieClip = new MovieClip();
    addChild(mc2);
    //clear the fullLoader—this is to help clear any full-size images that may already be on the stage (they have two options, click a new thumbnail, or close the current full-size image
    fullLoader.unload();
    //re-create the fullLoader, if there was one cleared
    fullLoader = new Loader();
    //load each full size image from its location per XML node's "source" deliniated by mc.ldr's "source" attribute
    fullLoader.load(new URLRequest(event.target.name));
    //add the loder to the container
    mc2.addChild(fullLoader);
    //create new instance of second loading loop
    mc2.fsloop = new loadingLoop2();
    //add the second loading loop to the container
    mc2.addChild(mc2.fsloop);
    //for each container, add a drop shadow
    mc2.filters = [fullDShadow];
    //set the container's position center on stage, making it the size of the loading loop until the full-size images are loaded
    mc2.x = (stage.stageWidth - mc2.fsloop.width) * 0.5;
    mc2.y = (stage.stageHeight - mc2.fsloop.height) * 0.5;
    //place container on stage
    addChild(mc2);
    //fade in each container
    fadeTween = new Tween(mc2, "alpha", None.easeNone, 0, 1, 0.5, true);
    //check to see if the image has been loaded completely—this is to make sure the image's attributes can be used to re-center on stage and remove fsloop
    fullLoader.contentLoaderInfo.addEventListener(Event.COMPLETE, removeLoop2);
    //set field to multiline so that the <br> HTML tags can be used
    imageTextField.multiline = true;
    //add text frame for description
    imageTextField.x = 10;
    imageTextField.y = 125;
    //for each full-size image, load the description text from the XML child nodes
    for (var j:int = 0; j < xmlList.length(); j++) {
        //check to make sure that the correct full-size image matches the text node when clicked
        if (xmlList[j].attribute("source") == event.target.name) {
            //add each child node as a separate line in the same text field; added HTML tags for CSS application
            imageTextField.htmlText = "<p><span class = 'projectName'>" + xmlList[j].child("projectName") + "</span><br>"
            + "<span class = 'toolsUsed'>" + xmlList[j].child("toolsUsed") + "</span><br><br>"
            + xmlList[j].child("projText") + "<br>"
            + "<span class = 'liveURL'>" + xmlList[j].child("liveURL") + "</span></p>";
    function removeLoop2(event:Event):void {
        //re-position the container to center on stage to accomodating the full sized image
        mc2.x = (stage.stageWidth - fullLoader.width) * 0.5;
        mc2.y = (stage.stageHeight - fullLoader.height) * 0.5;
        //Add the text field
        addText();
        //hide instructional text, already on stage
        galleryInfo_mc.alpha = 0;
        //remove the full-sized loading loop
        event.target.loader.parent.removeChild(MovieClip(event.target.loader.parent).loadingLoop2 );
    //set up full-size images to "self close" by clicking on it
    mc2.addEventListener(MouseEvent.CLICK, clearStage);
The second error gets thrown when the container is clicked and the function clearStage is executed, which does not remove the container:
"TypeError: Error #2007: Parameter child must be non-null.
    at flash.display::DisplayObjectContainer/removeChild()
    at artGallery_v1_5_fla::MainTimeline/clearStage()"
This is the clearStage function:
//Allow full-size image to "self close" by clikcing on it and removing the text description
function clearStage(event:MouseEvent):void {
    //clear the fullLoader
    fullLoader.unload();
    //remove the container
    event.target.parent.removeChild(MovieClip(event.target.parent).mc2);
   //remove the text field
    removeChild(imageTextField);
    //show the instructional text that is on-stage
    galleryInfo_mc.alpha = 1;
Help!