Ti ripeto la soluzione di prima integrandola in rosso nel tuo codice

codice:
var my_pb:mx.controls.ProgressBar;
var my_ldr:mx.controls.Loader;
var my_tree:mx.controls.Tree;
my_tree.setStyle("backgroundColor", "0xD02366");
my_tree.setStyle("color", "0xFFFFFF");
my_tree.setStyle("borderStyle", "none");
my_tree.setStyle("rollOverColor", "0xFFFF00");
my_tree.setStyle("selectionColor", "0x00FF00");
my_tree.setStyle("depthColors", [0xD02366]);
my_tree.setStyle("textIndent", 0);
my_tree.setStyle("indentation", 2);
my_tree.setStyle("selectionDuration", 0);
my_tree.vScrollPolicy = 'on';
my_tree.setStyle("defaultLeafIcon", "nullicon");
'my_tree.setStyle("folderOpenIcon", "nullicon");';
'my_tree.setStyle("folderClosedIcon", "nullicon");';
'my_tree.setStyle("disclosureClosedIcon", "nullicon");';
'my_tree.setStyle("disclosureOpenIcon", "nullicon");';
my_pb._visible = false;
my_pb.indeterminate = true;
my_pb.setSize(160, 30);
my_pb.source = my_ldr;
my_pb.mode = "polled";
var pbListener:Object = new Object();
pbListener.complete = function(evt:Object) {
	evt.target._visible = false;
};
my_pb.addEventListener("complete", pbListener);
var treeDP_xml:XML = new XML();
treeDP_xml.ignoreWhite = true;
treeDP_xml.onLoad = function(success:Boolean) {
	if (success) {
		my_tree.dataProvider = this.firstChild;
	}
};
treeDP_xml.load("gallery_tree.xml");
var treeListener:Object = new Object();
treeListener.change = function(evt:Object) {
	var treeNode:XMLNode = evt.target.selectedItem;
	if (evt.target.getIsBranch(treeNode)) {
		evt.target.setIsOpen(treeNode, !evt.target.getIsOpen(treeNode), true);
	} 
	if (treeNode.attributes.src != undefined) {
		my_pb._visible = true;
		sfondo._visible = false;
		my_ldr.load(treeNode.attributes.src);
		my_txt.text = treeNode.attributes.title;
	}
};
my_tree.addEventListener("change", treeListener);