flash as3错误＃1009：无法访问空对象引用的属性或方法
flash as3 Error #1009: Cannot access a property or method of a null object reference

Error #1009: Cannot access a property or method of a null object reference.
at sto()[C:\Users\dani\Desktop\folder (2)\sto.as:11]
at runtime::ContentPlayer/loadInitialContent()
at runtime::ContentPlayer/playRawContent()
at runtime::ContentPlayer/playContent()
at runtime::AppRunner/run()
at ADLAppEntry/run()
at global/runtime::ADLEntry()

我用flash as3创建一个按钮。 你们能帮我吗

package  {

import flash.display.MovieClip;
import flash.events.MouseEvent;


public class sto extends MovieClip {


    public function sto() {

     start.addEventListener(MouseEvent.CLICK,kdown);//start is the instance name of a movieclip

            }
               function kdown(event:MouseEvent):void{
        nextFrame();
       }

}

}

我认为问题是start.addEventListener（MouseEvent.CLICK，kdown）; 但我不知道该怎么办