TypeError: Error #1009: Cannot access a property or method of a null object reference. Can't Use stage.addEventListener - actionscript-3

Question

If anyone can help me??
I have trouble when I try to log into my main game.
I have a error like this:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at MenuMain$iinit()
at DocumentMain/gamemain()
at DocumentMain/mouseClick1()
And this is my script in MainGame :
package {
import flash.display.MovieClip;
import flash.events.MouseEvent;
import flash.events.KeyboardEvent;
import flash.events.Event;
import flash.media.SoundChannel;
import flash.geom.Point;
import flash.display.Sprite;
public class MenuMain extends MovieClip {
public var leftPressed:Boolean = false;
public var rightPressed:Boolean = false;
public var upPressed:Boolean = false;
public var downPressed:Boolean = false;
public var leftBumping:Boolean = false;
public var rightBumping:Boolean = false;
public var upBumping:Boolean = false;
public var downBumping:Boolean = false;
public var leftBumpPoint:Point = new Point(-36, -15);
public var rightBumpPoint:Point = new Point(36, -15);
public var upBumpPoint:Point = new Point(0, -120);
public var downBumpPoint:Point = new Point(0, 0);
public var scrollX:Number = 0;
public var scrollY:Number = 650;
public var xSpeed:Number = 0;
public var ySpeed:Number = 0;
public var speedConstant:Number = 4;
public var frictionConstant:Number = 0.9;
public var gravityConstant:Number = 1.8;
public var jumpConstant:Number = -35;
public var maxSpeedConstant:Number = 18;
public var doubleJumpReady:Boolean = false;
public var upReleasedInAir:Boolean = false;
public var animationState:String = "still";
public function MenuMain(){
stage.addEventListener(KeyboardEvent.KEY_DOWN, keyDownHandler);
stage.addEventListener(KeyboardEvent.KEY_UP, keyUpHandler);
stage.addEventListener(Event.ENTER_FRAME, loop);}
public function loop(e:Event):void{
if(back.collision.hitTestPoint(player.x + leftBumpPoint.x, player.y + leftBumpPoint.y, true)){
leftBumping = true;
} else {
leftBumping = false;
}
if(back.collision.hitTestPoint(player.x + rightBumpPoint.x, player.y + rightBumpPoint.y, true)){
rightBumping = true;
} else {
rightBumping = false;
}
if(back.collision.hitTestPoint(player.x + upBumpPoint.x, player.y + upBumpPoint.y, true)){
upBumping = true;
} else {
upBumping = false;
}
if(back.collision.hitTestPoint(player.x + downBumpPoint.x, player.y + downBumpPoint.y, true)){
downBumping = true;
} else {
downBumping = false;
}
if(leftPressed){
xSpeed -= speedConstant;
player.scaleX = -1;
} else if(rightPressed){
xSpeed += speedConstant;
player.scaleX = 1;
}
if(leftBumping){
if(xSpeed < 0){
xSpeed *= -0.5;
}
}
if(rightBumping){
if(xSpeed > 0){
xSpeed *= -0.5;
}
}
if(upBumping){
if(ySpeed < 0){
ySpeed *= -0.5;
}
}
if(downBumping){ //Jika mengenai tanah/dasar
if(ySpeed > 0){
ySpeed = 0; //set y speed ke nol
}
if(upPressed){ //dan jika arah keatas ditekan
ySpeed = jumpConstant; //set y speed ke jump constant
}
//DOUBLE JUMP
if(upReleasedInAir == true){
upReleasedInAir = false;
}
if(doubleJumpReady == false){
doubleJumpReady = true;
}
} else { //Jika mengenai tanah/dasar
ySpeed += gravityConstant; //akslerasi kebawah
//DOUBLE JUMP
if(upPressed == false && upReleasedInAir == false){
upReleasedInAir = true;
}
if(doubleJumpReady && upReleasedInAir){
if(upPressed){ //dan jika arah keatas ditekan
doubleJumpReady = false;
ySpeed = jumpConstant; //set y speed ke jump constant
}
}
}
if(xSpeed > maxSpeedConstant){ //gerak kanan
xSpeed = maxSpeedConstant;
} else if(xSpeed < (maxSpeedConstant * -1)){ //gerak kiri
xSpeed = (maxSpeedConstant * -1);
}
xSpeed *= frictionConstant;
ySpeed *= frictionConstant;
if(Math.abs(xSpeed) < 0.5){
xSpeed = 0;
}
scrollX -= xSpeed;
scrollY -= ySpeed;
back.x = scrollX;
back.y = scrollY;
sky.x = scrollX * 0.2;
sky.y = scrollY * 0.2;
if( ( leftPressed || rightPressed || xSpeed > speedConstant || xSpeed < speedConstant *-1 ) && downBumping){
animationState = "jalan";
} else if(downBumping){
animationState = "still";
} else {
animationState = "jump";
}
if(player.currentLabel != animationState){
player.gotoAndStop(animationState);
}
}
public function keyDownHandler(e:KeyboardEvent):void
{
if(e.keyCode == 37){
leftPressed = true;
} else if(e.keyCode == 39){
rightPressed = true;
} else if(e.keyCode == 38){
upPressed = true;
} else if(e.keyCode == 40){
downPressed = true;
}
}
public function keyUpHandler(e:KeyboardEvent):void
{
if(e.keyCode == 37){
leftPressed = false;
} else if(e.keyCode == 39){
rightPressed = false;
} else if(e.keyCode == 38){
upPressed = false;
} else if(e.keyCode == 40){
downPressed = false;
}
}
}
}
And this is myfile download here :
https://www.dropbox.com/s/g36hfykvebiu0sw/MyGame.rar
Thanks for help

Answer 1

You cannot access the stage because it is being called before the object is actually added to the stage. This usually happens when you try to access the stage in constructors. Use the AddedToStage Event instead. Try this:
public function MenuMain(){
addEventListener(Event.ADDED_TO_STAGE, onAddedToStage);
}
public function onAddedToStage(e:Event):void{
stage.addEventListener(KeyboardEvent.KEY_DOWN, keyDownHandler);
stage.addEventListener(KeyboardEvent.KEY_UP, keyUpHandler);
stage.addEventListener(Event.ENTER_FRAME, loop);
removeEventListener(Event.ADDED_TO_STAGE, onAddedToStage);
}