TypeError: Error #1009- help needed

Question

Hi,
I'm working on a school project, which is to say I'm still pretty new to AS3.
I keep receiving error 1009 which seems to point to the  problematic "theory" section. I've checked my label & instance names  numerous times- everything looks fine. What else might it be?
TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at Untitled_fla::MainTimeline/frame70()
The buttons fade into view over the course of 5 frames.. so they are on the stage but invisible starting at frame 65, and by frame 70 they are in the same spot and fully visible & supposed to be functional.
I have the actionscript on a separate layer on frame 70, this is what I have-
stop();
theory_btn.addEventListener(MouseEvent.CLICK,playtheory);
function playtheory(event:MouseEvent):void{
    gotoAndPlay("theory");
history_btn.addEventListener(MouseEvent.CLICK,playhistory);
function playhistory(event:MouseEvent):void{
    gotoAndPlay("history");
future_btn.addEventListener(MouseEvent.CLICK,playfuture);
function playfuture(event:MouseEvent):void{
    gotoAndPlay("future");
credits_btn.addEventListener(MouseEvent.CLICK,playcredits);
function playcredits(event:MouseEvent):void{
    gotoAndPlay("credits");
I've checked all of my labeling mulitple times- all names and instances are correct and in their proper places.. Unless it's a mindblowingly obvious mistake and I'm just not catching it.
I'd like to upload my .fla, but it's too large of a file. I will happily email it if anyone is willing to take a peek.

Answer 1

I may be wrong, but for the original problem I am guessing that the following of what I listed was the problem...
- is animated into place but is not assigned instance names in every keyframe for it
You mentioned frame 70 is where alpha = 1, which has me thinking you animated them into view but did not name them in the first frame(s).  That lack of a name will get inherited at the other end of the tween.
For your new problem, if your home page is on frame 75, and your code for the home button is on frame 76, you have to stop at frame 76 for the code to be assigned, otherwise, going to another frame will leave it without that code getting assigned for the home button.
What you can do is have the home button and its code in frame 75, but make it invisible there, and in your other function for the other buttons, turn it visible.
I tried to show the code but the editor is playing games and deleting stuff on me as I add it.