TypeError: Error #1009 (Help Please?)

I'm new with the whole ActionScripting Thing. :( So I truly appreciate it if someone can explain to me as simply as possible. :D

I keep getting the errors below when I click to go to another page. Please help me out if I'm doing something wrong. ^^

TypeError: Error #1009: Cannot access a property or method of a null object reference. at FlashDoc_fla::MainTimeline/followBall_a() TypeError: Error #1009: Cannot access a property or method of a null object reference. at FlashDoc_fla::MainTimeline/rotate_a()

Codes are as follow...

stage.addEventListener(Event.ENTER_FRAME,rotate_a);
function rotate_a (e:Event){
 var theX:int = mouseX - eye_ball_icon.x;
 var theY:int = (mouseY - eye_ball_icon.y) * -1;
 var angle = Math.atan(theY/theX)/(Math.PI/180);
 if (theX<0开发者_C百科) {
  angle += 180;
 }
 if (theX>=0 && theY<0) {
  angle += 360;
 }
 eye_text.text = angle;
 eye_ball_icon.rotation = (angle*-1) + 90;
}


stage.addEventListener(Event.ENTER_FRAME,followBall_a);
function followBall_a(event:Event):void {
 var dx:int = eye_ball_icon.x - mouseX;
 eye_ball_icon.x -= dx / 20;
 eye_ball_icon.y=530;

 if (eye_ball_icon.x < 150){eye_ball_icon.x = 150};
 if (eye_ball_icon.x > 850){eye_ball_icon.x = 850};
}


stop();
about_icon.addEventListener(MouseEvent.CLICK,iconpage);
function iconpage(event:MouseEvent) { gotoAndPlay(3); }

works_icon.addEventListener(MouseEvent.CLICK,workspage);
function workspage(event:MouseEvent) { gotoAndPlay(4); }

contact_icon.addEventListener(MouseEvent.CLICK,contactpage);
function contactpage(event:MouseEvent) { gotoAndPlay(5); }

Try this rotate_a method:

function rotate_a (e:Event){
if(currentFrame==1){
    var theX:int = mouseX - eye_ball_icon.x;
    var theY:int = (mouseY - eye_ball_icon.y) * -1;
    var angle = Math.atan(theY/theX)/(Math.PI/180);
    if (theX<0) {
        angle += 180;
    }

    if (theX>=0 && theY<0) {
        angle += 360;
    }
    eye_text.text = angle;
    eye_ball_icon.rotation = (angle*-1) + 90;
}}

It checks to see if you are on frame 1 before trying to do anything with the objects on frame 1. I'm assuming that you have this code in the first frame, and that eye_text and eye_ball_icon are on the first frame.