Frame switching using buttons help please

Can anyone help me please with the following problem, as I’m a novice and don’t have much experience using Flash?

From Frame 1 to 15 I have a button movieclip which will when clicked on switch to frame 16, but a different button exist on frame 16. This button on frame 16 should have the same functionality as the one at the beginning when its clicked on but switch to frame 24. For some reason I’m getting a:

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

Here’s my code for the button at the beginning which works:

startbutton.addEventListener(MouseEvent.CLICK, fl_MouseClickHandler);

function fl_MouseClickHandler(event:MouseEvent):void
{
gotoAndStop(16);
stage.removeEventListener(Event.ENTER_FRAME, update);
}

stage.addEventListener(Event.ENTER_FRAME, update);
function update(evt:Event):void
{
startbutton.x = stage.mouseX - startbutton.width * 0.3;
startbutton.y = stage.mouseY - startbutton.height * 0.3;
}

Now here’s my code for the second button:

ForSaleButton.addEventListener(MouseEvent.CLICK, ClickForSale);

function ClickForSale(event:MouseEvent):void
{
gotoAndStop(24);
}

Thanks for your help in this matter. Your help has been much appreciated.

:rd::batman::sonic::jamesbond::toad::mr_t::luigi::mario::bobafett::samus_v2::samuelljackson::megaman_x::vader::megaman_classic::megaman::kir::stormtrooper::yoda: