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

Hello there everyone. I'm having a problem with my action script. I'm trying to create a quiz for one of my classes and I'm at a certain point where I can go no further. Everytime I try and preview the file, and error message appears in the output window that says

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

          at Bosco_fla::MainTimeline/frame1()[Bosco_fla.MainTimeline::frame1:9]

Here is my code:

stop();

takequizbtn.addEventListener(MouseEvent.CLICK, fl_ClickToGoToAndStopAtFrame);

function fl_ClickToGoToAndStopAtFrame(event:MouseEvent):void

{

          gotoAndStop(2);

}

easy_btn.addEventListener(MouseEvent.CLICK, fl_ClickToGoToAndStopAtFrame_2);

function fl_ClickToGoToAndStopAtFrame_2(event:MouseEvent):void

{

          gotoAndStop(3);

}

If anyone can help me it will be appreciated. Thank You!