无法移动框架

Question

我做了一个捕手游戏，当时间耗尽时遇到麻烦无法移动框架

我已经做过的事情：

function addMc()
{
    var NewMc = new MC();
    NewMc.y = Math.ceil(Math.random() * 500);
    NewMc.x = 50;
    addChildAt(NewMc,1);
    pelotas.push(NewMc);
    NewMc.addEventListener(Event.ENTER_FRAME, dropMc);
}

function dropMc(e:Event)
{
    var b:MC = MC(e.target);
    b.x +=  10;
    if (b.x > 900)
    {
        eliminasiMC(b);
    }
}

stage.addEventListener(Event.ENTER_FRAME, catcher);

function catcher(e:Event)
{
    catcherMC.y = mouseY;
    for (var i:int=0; i<pelotas.length; i++)
    {
        if (catcher.hitTestObject(pelotas[i]))
        {
            eliminasiMC(pelotas[i]);
            point++;
            txtPoint.text = String(point);
        }
    }
}

function eliminasiMC(B)
{
    B.removeEventListener(Event.ENTER_FRAME, dropMc);
    removeChild(B);
}

var tTime:Number = 5;
var timer:Timer = new Timer(1000,tTime);
timer.addEventListener(TimerEvent.TIMER, countdown);
timer.start();
function countdown(event:TimerEvent)
{
    txtTime.text=String((tTime)-timer.currentCount);
    if (txtTime.text == "0")
    {
        stage.removeEventListener(Event.ENTER_FRAME, catcher);
        timer.stop();
        timer.removeEventListener(TimerEvent.TIMER, countdown);
        gotoAndStop("nextFrame"); // HERE MY PROBLEM
    }
}

但是发生了这个错误：

TypeError: Error #1009: Cannot access a property or method of a null object reference.  at GEBUGENDE_fla::MainTimeline/addBabi()[GEBUGENDE_fla.MainTimeline::frame80:18]    at Function/http://adobe.com/AS3/2006/builtin::apply()  at SetIntervalTimer/onTimer()   at flash.utils::Timer/_timerDispatch()  at flash.utils::Timer/tick()