ActionScript 3.0 :: Slideshow (Fade In / Out) With Multiple Buttons

I'm working on a slide show which I would like work automatically (fade in and out) as well as being able to be prompted to switch via buttons... I know this is a super simple thing, but I cannot get all the buttons to work. The buttons are copies of the same instance with different instance names and are located at the 3 respective frames in timeline where the pieces of slideshow should fade in/out. I get the first set of buttons to work:

import flash.events.MouseEvent;
controls.bttn1.addEventListener (MouseEvent.CLICK, clickSection1);
function clickSection1 (evtObj:MouseEvent) {
// trace shows what's happening in the output window
trace ("The Button 1 was clicked!")

But when I add more buttons that are located later on in a timeline, I get the following error:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at INDEX_TEST_r3_fla::MainTimeline/frame1()