Dudes this is pissing me off

1

my game timer code is :

var count:Number = 36;


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


myTimer.addEventListener(TimerEvent.TIMER,  countdown);

myTimer.start();

function  countdown(event:TimerEvent):void {
    counter.text =  String((count)-myTimer.currentCount);//<----- debugger is showing  error here

    if(count <= 0){
         myTimer.removeEventListener(TimerEvent.TIMER, countdown);
         myTimer.stop();           
}
if (counter.text == String("0")) {
    gotoAndPlay(426);
   }
}

and sometimes in middle of playing it shows me this error :

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

What to do, please answer. Thanks
Aleksandar