SImple Coloring Book: Error #1009: Cannot access a property or method of a null object reference - actionscript-3

Question

I am creating a simple flash coloring book and am not very familiar with as3 programming language.
I entered the following code,and when I attempted to press the back button in the test movie I got that error.
stop();
back_btn.addEventListener(MouseEvent.CLICK, GoToChooseA);
function GoToChooseA(event:MouseEvent):void
{
gotoAndStop("Choose");
}
color_scroll.mask = myMask;
var goY: Number = color_scroll.y;
stage.addEventListener(Event.ENTER_FRAME, scrollManage);
function scrollManage(Event): void {
color_scroll.y += (goY - color_scroll.y) / 20;
}
up_btn.addEventListener(MouseEvent.MOUSE_DOWN, scrollUP);
down_btn.addEventListener(MouseEvent.MOUSE_DOWN, scrollDown);
function scrollUP(MouseEvent): void {
goY += 20;
}
function scrollDown(MouseEvent): void {
goY -= 20;
}
*
It seems to indicate the error is here
color_scroll.y += (goY - color_scroll.y) / 20;
But I'm really bummed because I'm not really sure how to proceed from there.

Answer 1

Whenever you gotoAndStop() to a different keyframe, your current frame is invalidated and all its members destroyed. Listeners persist, if they are attached to an object that persists. So, right after you call GoToChooseA(), your color_scroll is destroyed, and then the listener attached to stage is called and tries to modify a destroyed object, there goes your 1009. The solution is either manually remove the event listeners "scrollManage", "scrollUp", "scrollDown" before you change the frame, at least "scrollManage" because it's attached to stage, or stop using frames altogether, but even then you'll have to control your event listeners.

Answer 2

You could add some logic to your function to check if you are in the right frame and then proceed. I am not familiar with frames so the condition would be something like this._currentframe == 2 or timeline.currentFrame == 2.
function scrollManage(Event): void {
if ( condition ) {
color_scroll.y += (goY - color_scroll.y) / 20;
}
}
If you are not at the right frame (in my example that is frame 2), the function does not execute any code.
This error means you are trying to modify something that is no longer existent.