Error #1009, How to do function not null?

programming flash
1

Hi, so i have some problem i wrote a code with functions but when i testing movie i got some error like “TypeError: Error #1009: Cannot access a property or method of a null object reference.”.
Who can help me to solve this problem?
There is code:

import flash.events.MouseEvent;

stop();
function kontaktaiOver(event:MouseEvent):void {
event.target.gotoAndPlay(“kont_iejimas”);
}
function kontaktaiOut(event:MouseEvent):void {
event.target.gotoAndPlay(“kont_isejimas”);
}

kontaktaiButt.addEventListener(MouseEvent.ROLL_OVER, kontaktaiOver);
kontaktaiButt.addEventListener(MouseEvent.ROLL_OUT, kontaktaiOut);