将变量强制转换为电影剪辑

如何将壁虎对象转换为电影剪辑?

function finish(boxname, arrayname:Array):void {

for each (var item:String in arrayname) {
    trace(boxname+"_"+item);
    var gecko:MovieClip = (boxname+"_"+item) as MovieClip ;
    trace(typeof(gecko));
    gecko.gotoAndPlay("glow");
    }

  }

出现以下错误:

high_hsymbol_1
object
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at quizz_fla::MainTimeline/finish()
at quizz_fla::MainTimeline/dropIt()