Cannot access a property or method of a null object reference.

Hi there everyone, I've been working on a multimedia project for I.T course.

I have no experience in actionscript 3 whatsoever.

I keep getting this message in flash cs4 when I try to actions to the my menu buttons

"TypeError: Error #1009: Cannot access a property or method of a null object reference.

at Enigma_fla::MainTimeline/frame1()"

stop();

enter_btn.addEventListener(MouseEvent.CLICK,enter_btnClick);

home_btn.addEventListener(MouseEvent.CLICK,home_btnClick);

gallery_btn.addEventListener(MouseEvent.CLICK,gallery_btnClick);

depart_btn.addEventListener(MouseEvent.CLICK,depart_btnClick);

courses_btn.addEventListener(MouseEvent.CLICK,courses_btnClick);

interact_btn.addEventListener(MouseEvent.CLICK,interact_btnClick);

function enter_btnClick(ev:MouseEvent):void

{

          gotoAndPlay("enigmastart");

}

function home_btnClick(ev:MouseEvent):void

{

          gotoAndPlay("enigmastart");

}

function gallery_btnClick(ev:MouseEvent):void

{

          gotoAndPlay("galleria");

}

function depart_btnClick(ev:MouseEvent):void

{

          gotoAndPlay("department");

}

function courses_btnClick(ev:MouseEvent):void

{

          gotoAndPlay("courses");

}

function interact_btnClick(ev:MouseEvent):void

{

          gotoAndPlay("interact");

}

stop();