Error in the middle of the game?! xD

1

i runned debuger
(error shows sometimes, and sometimes not XD)


TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at test_fla::MainTimeline/countdown()[test_fla.MainTimeline::frame571:14]
    at flash.utils::Timer/_timerDispatch()
    at flash.utils::Timer/tick()

i must go on continue to continue play
debugger shows error

var count:Number = 60;

//2.
var myTimer:Timer = new Timer(1000,count);

//3.
myTimer.addEventListener(TimerEvent.TIMER, countdown);

//4.
myTimer.start();

//5.
function countdown(event:TimerEvent):void {
counter.text = String((count)-myTimer.currentCount);//<-------- HERE

if(counter.text == "0"){
gotoAndPlay(1);
}
}

Any help?! Thanks
Aleksandar