[AS3]gotoAndPlay not working

programming flash
1

hello everybody,
Plz help my gotoAndPlay function not working, I am getting this runtime error "[COLOR=Red]Error #1009: Cannot access a property or method of a null object reference.[/COLOR]"I understand its meaning, what is the reference that is null here , even keyframe 3 exists.

q1.addEventListener(MouseEvent.CLICK,clik);
q2.addEventListener(MouseEvent.CLICK,clik);
q3.addEventListener(MouseEvent.CLICK,clik);
q4.addEventListener(MouseEvent.CLICK,clik);

function clik(event:MouseEvent)
{ 
    nex.addEventListener(MouseEvent.CLICK, abc);
    function abc(event:MouseEvent)
    {
        if(Number(RBG.selectedData)==1)//RBG is my radiobuttongroup
        {
            mark=mark + num;
        trace("correct answer="+mark);
        
        }
        else 
        {   mark=mark-num;
            trace("Incorrect answer="+mark);
        }
        gotoAndPlay(3); 
    }