ActionScript 3.0 :: Next / Prev Frame - Opening Menu When Button Clicked

openMenu.addEventListener(MouseEvent.CLICK, openSubMenu);
function openSubMenu(event:MouseEvent):void {
this.openMenu.nextFrame();
} openMenu.closeMenu.addEventListener(MouseEvent.CLICK, closeSub);
function closeSub(event:MouseEvent):void {
this.openMenu.prevFrame();
} stop();

I've got the above code on my first frame, and was hoping that it would open a menu when a button was clicked, and once the button within the MC is clicked it would then close. Hence the nextFrame and prevFrame actions.

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