ActionScript 3错误＃1009空对象引用

Question

我在为我的flash项目构建的命令系统中遇到了一些错误。其中一个命令是介绍，它基本上重新启动项目，或者意味着（介绍从第一个场景的第0帧开始）。但是当它运行时，引用错误被抛出我已经确定为第21行（通过注释代码直到它不再出错），这是引入函数的thisRef.gotoAndPlay行。

这是我的代码：

package  {
    import flash.display.MovieClip;
    import flash.events.MouseEvent;
    import flash.system.fscommand;
    import flash.events.KeyboardEvent;
    import flash.media.Sound;

    public class Main extends MovieClip {

        /* The real frame number */
        public var realFrameNo:int = 0;

        /* Whether startup has been ran or not */
        public var startupRan:Boolean = false;

        /* Whether or not the beggining text is there */
        public var placeholderPresent:Boolean = true;

        /* Commands recognized by the computer [command:string,handler:void] */
        public var commandList = new Array(["intro",function(thisRef:Main) {
                                                gotoAndPlay(0,"Intro");
                                            }],
                                           ["exit",function(thisRef:Main) {
                                                fscommand("quit");
                                            }]);

        /* Helper functions */
        public function isAlphabetic(value:String):Boolean
        {
            var result:String = value.match(/[a-zA-Z]*/)[0];
            return value.length == result.length;
        }

        public function getSceneNo():int
        {
            return (realFrameNo > 0 ? Math.floor( (realFrameNo / stage.frameRate) ) : 0 );
        }

        public function getSceneName(sceneNo:int):String
        {
            switch(sceneNo)
            {
                default:
                    return "Intro";
                case 0:
                    return "Intro";
                case 1:
                    return "MainMenu";
            }
        }

        /* Main functions */
        public function Main() 
        {
            this.addEventListener('enterFrame',function() {
                if(!this.startupRan)
                    realFrameNo += 1;

                /* Main menu init */
                if(getSceneNo() == 1 && !this.startupRan && currentFrame == 2)
                {
                    stop();
                    this.startupRan = true;

                    /* Add keyboard events */
                    stage.addEventListener(KeyboardEvent.KEY_DOWN,function(event:KeyboardEvent){
                        var letter:String = String.fromCharCode(event.charCode).toUpperCase();

                        if(event.keyCode == 8)
                            updateMonitor("");
                        else if(event.keyCode == 13)
                            evalInput();
                        else if(isAlphabetic(letter))
                            updateMonitor(letter);
                    });

                    /* Add button events */
                    btnQ.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("Q"); });
                    btnW.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("W"); });
                    btnE.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("E"); });
                    btnR.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("R"); });
                    btnT.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("T"); });
                    btnY.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("Y"); });
                    btnU.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("U"); });
                    btnI.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("I"); });
                    btnO.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("O"); });
                    btnP.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("P"); });
                    btnA.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("A"); });
                    btnS.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("S"); });
                    btnD.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("D"); });
                    btnF.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("F"); });
                    btnG.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("G"); });
                    btnH.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("H"); });
                    btnJ.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("J"); });
                    btnK.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("K"); });
                    btnL.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("L"); });
                    btnZ.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("Z"); });
                    btnX.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("X"); });
                    btnC.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("C"); });
                    btnV.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("V"); });
                    btnB.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("B"); });
                    btnN.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("N"); });
                    btnM.addEventListener(MouseEvent.CLICK,function(){ updateMonitor("M"); });
                    btnDel.addEventListener(MouseEvent.CLICK,function(){ updateMonitor(""); });
                    btnEnter.addEventListener(MouseEvent.CLICK,function(){ evalInput(); });
                    btnReturn.addEventListener(MouseEvent.CLICK,function(){ evalInput(); });
                }


            });
        }

        public function updateMonitor(letter:String)
        {
            if(this.placeholderPresent) //Remove placeholder
                lblInput.text = letter;
            else if(letter == '' || letter == null) //Backspace
            {
                lblInput.text = lblInput.text.substr(0,lblInput.text.length-1);
            }
            else //Append text
                lblInput.appendText(letter);

            if(lblInput.text == "")
            {
                lblInput.text = "Type something to begin..";
                this.placeholderPresent = true;
            }
            else
                this.placeholderPresent = false;
        }

        public function evalInput():void
        {
            if(this.placeholderPresent) //Don't eval the starting text
                return;

            lblOutput.text = "Scanning for commands...";

            for(var i:int = 0; i < this.commandList.length; i++)
            {
                if(lblInput.text.toLowerCase() == this.commandList[i][0])
                {
                   this.commandList[i][1](this);
                   lblOutput.appendText("\nCommand ran.");
                   return;
                }
            }
            lblOutput.appendText("\nNo command recognized.");
        }
    }
}

完整堆栈跟踪：

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at Main/evalInput()[H:\Year 2\Units\Unit 43 - Multimedia Design\Flash Application\Main.as:144]
    at Function/<anonymous>()[H:\Year 2\Units\Unit 43 - Multimedia Design\Flash Application\Main.as:71]