Error #2025: ActionScript problems with programing - actionscript-3

Question

The problem is that every time I try to go on to the next scene backStory
it doesn't remove the child. I want to make it so that when the counter goes to
4 it will take it out and put in a new scene. Is there a way to fix this issue? I don't understand how I can switch scenes
I am creating a game and I get this error :
ArgumentError: Error #2025: The supplied DisplayObject must be a
child of the caller.
at flash.display::DisplayObjectContainer/removeChild()
at Runner/onEnterFrame()
package {
import flash.events.Event;
import flash.events.MouseEvent;
import flash.events.KeyboardEvent;
import flash.ui.Keyboard;
import flash.display.MovieClip;
import flash.text.TextField;
import flash.text.TextFormat;
public class Runner extends MovieClip {
public var startPage:StartPage = new StartPage();
public var backStory1:BackStory1 = new BackStory1();
public var water:Water = new Water();
public function Runner() {
addChild(startPage);
stage.addEventListener(Event.ENTER_FRAME, onEnterFrame);
}
function onEnterFrame(event:Event):void{
if(startPage.endStartPage == true){
removeChild(startPage);
addChild(backStory1);
startPage.endStartPage == false;
}
if(backStory1.backStory1End == true){ //in backstory1 the bool backstory1end is suppose to be true but it doesnt get to that point
removeChild(backStory1);
addChild(water);
startPage.endStartPage == false;
}
}
}
}
Backstory class
package {
import flash.events.Event;
import flash.events.MouseEvent;
import flash.events.KeyboardEvent;
import flash.ui.Keyboard;
import flash.display.MovieClip;
import flash.text.TextField;
import flash.text.TextFormat;
public class BackStory1 extends MovieClip {
var backStory1End:Boolean = false;
var count:int = 0;
public function BackStory1() {
backStory1Text.text = "sssherro";
if (stage)
{
init(null);
}
else
{
addEventListener(Event.ADDED_TO_STAGE,init);
}
function init(e:Event):void
{
removeEventListener(Event.ADDED_TO_STAGE, init);
nextButton.addEventListener(MouseEvent.MOUSE_DOWN,onButtonClick);
}
function onButtonClick(event:MouseEvent):void{
count++;
if(count == 1){
backStory1Text.text = "awesome1";
//backStory1End= true;
}
else if(count == 2){
backStory1Text.text = "awesome2";
}
else if(count == 3){
backStory1Text.text = "awesome3 leave game press";
}
else if(count == 4){
//backStory1Text.text = (String)(counter);
backStory1End = true;
}
}
}
}
}

Answer 1

Following line is wrong.
startPage.endStartPage == false;
Fix:
startPage.endStartPage = false;