Error #2007: Parameter text must be non-null

1

Hi there,
My .swf file show the following error

TypeError: Error #2007: Parameter text must be non-null.
at flash.text::TextField/set text()
at Xml/xmlDone()[C:\Users\TaiLT01319\Desktop\xml\Xml.as:62]
at Xml/onStopWatch()[C:\Users\TaiLT01319\Desktop\xml\Xml.as:74]
at flash.utils::Timer/_timerDispatch()
at flash.utils::Timer/tick()

And this here is as3 code:

package  {
    
    import flash.display.*;
    import flash.events.*;
    import flash.net.URLRequest;
    import flash.net.URLLoader;
    import flash.xml.XMLDocument;
    import fl.transitions.Tween;
    import fl.transitions.easing.*;
    import flash.utils.Timer;
    
    public class Xml extends MovieClip {
        
        private var myXML:XML = new XML();
        private var XML_URL:String = "flash_game.xml";
        private var mangThuTu:Array = new Array();
        private var mangChu:Array = new Array();
        
        private var myXMLURL:URLRequest = new URLRequest(XML_URL);
        private var myLoader:URLLoader = new URLLoader(myXMLURL);
        
        private var _timer:Timer;
        private var timeAuto = 5000;
        
        public function Xml() {
            // constructor code
            myLoader.addEventListener("complete", xmlLoaded);
        }
        
        private var khungChu:Array = new Array();
        private var myTween:Array = new Array();
        
        public function xmlLoaded(event:Event)
        {
            myXML = XML(myLoader.data);
            var xmlDoc:XMLDocument = new XMLDocument();
            xmlDoc.ignoreWhite = true;
            var menuXML:XML = XML(myLoader.data);
            xmlDoc.parseXML(menuXML.toXMLString());
            
            for each (var Data:XML in myXML..Data)
            {      
                var ID:String = Data.ID.toString();
                var Alphabet:String = Data.Alphabet.toString();
                mangThuTu.push(ID);
                mangChu.push(Alphabet);
            }
            
            xmlDone();
        }
        
        public function xmlDone()
        {
            for (var i = 0; i<4 ; i++)
            {
                var chuDuocChon:String = mangChu[Math.floor(Math.random()*mangChu.length)];    
                mangChu.splice(mangChu.indexOf(chuDuocChon,1),1);
                khungChu* = new chu();
                addChild(khungChu*);
                //khungChu*.x = 50*i;
                khungChu*.y = 50*i;
                khungChu*.txt.text = chuDuocChon;
                myTween* = new Tween(khungChu*, "x", Elastic.easeInOut, 20, 400, 2, true);
                khungChu*.addEventListener(MouseEvent.CLICK, answer);
            }
            _timer = new Timer(timeAuto);
            _timer.addEventListener(TimerEvent.TIMER, onStopWatch);
            _timer.start();
        }
        
        public function onStopWatch(e:Event)
        {
            clearStage();
            xmlDone();
        }
        
        public function clearStage()
        {
            for (var i = 0; i<4 ; i++)
            {
                khungChu*.visible = false;
            }    
        }
        
        public function answer(evt:MouseEvent)
        {
            evt.target.parent.visible = false; // goi bo cua thang bi an
        }
    }
    
}

I am trying but i don’t know how to fix this error. Does anyone have any ideas? Thanks for reading!