AS3.0 无法访问空对象引用的属性或方法 ,主要是实现多文件的上传，求大神指点！

var FR:FileReferenceList = new FileReferenceList();var Frl:Array;var total:Number=0;var Frist:Number=0;botton1.addEventListener(MouseEvent.CLICK, fl_down);问题出现在这里。。。无法访问空对象引用的属性或方法 function onSelected(e:Event):void{ var frl:FileReferenceList=e.target as FileReferenceList; for each(var f:FileReference in frl.fileList){ Frl.push(f); trace(f.name); } total=Frl.length; Frist=1; upLoadFile();}function upLoadFile(i:int=0):void{ var fb:FileReference=Frl[i]; if(fb==null) return; fb.addEventListener(Event.COMPLETE,onUploadComplete);}function onUploadComplete(e:Event):void{ Frl.shift(); upLoadFile(); Frist++;} function onCancel(e:Event){ trace("2");}function fl_down(e:MouseEvent){ FR.browse(); FR.addEventListener(Event.SELECT,onSelected); FR.addEventListener(Event.CANCEL,onCancel);}