چرا در این کد trace من null را برمیگرداند

[Ali Mehrzad]

private var xmlEPG1:XML;
private var xmlStrings:String;
...
private function inform_of_shahid(){
xmlLoader("EPG.xml");
trace("xmlStrings: "+xmlStrings);
}


private function xmlLoader(urXmlAdress:String){
var urXml:URLRequest;
urXml = new URLRequest(urXmlAdress);
ulXml1 = new URLLoader(urXml);
ulXml1.addEventListener(Event.COMPLETE, xmlLoaded1);
ulXml1.load(urXml);
}
private function xmlLoaded1(event:Event){
xmlEPG1 = new XML(ulXml1.data);
xmlStrings = xmlEPG1.toString();
}