Error 1009 : Cannot access a property of method of a null object reference - actionscript-3

Question

I don't understand what is going on
This is my Main.as
package {
import flash.display.MovieClip;
import flash.events.MouseEvent;
public class Main extends MovieClip {
public var pirkles:Circles = new Circles()
public function Main() {
gotoAndStop(1)
playbtn.addEventListener(MouseEvent.CLICK, playscreen)
}
public function playscreen(event:MouseEvent):void {
gotoAndStop(2)
addChild(pirkles)
}
}
}
And this is my Circles.as
package {
import flash.display.MovieClip
import flash.events.Event;
import flash.events.KeyboardEvent;
import flash.ui.Keyboard
import flash.events.MouseEvent;
public class Circles extends MovieClip{
public function Circles():void {
stage.addEventListener(KeyboardEvent.KEY_DOWN, MOVE)
this.y = 175
this.x = 10
}
public function MOVE(event:KeyboardEvent):void {
if (event.keyCode == Keyboard.RIGHT) {
this.x = this.x+10
}
else if (event.keyCode == Keyboard.LEFT) {
this.x = this.x-10
}
else if (event.keyCode == Keyboard.UP) {
this.y = this.y-10
}
else if (event.keyCode == Keyboard.DOWN) {
this.y = this.y+10
}
}
}
}
Now I get an error that says there is a problem at line 11 of my Circles.as and at line 8 of my Main.as. However, at those lines I don't understand what is causing the problem. I added a event listener at line 11, but when I take it out it works. Also, at line 8, I Just defined a variable.

Answer 1

you can't access the stage in a class constructor.
So the line
stage.addEventListener(KeyboardEvent.KEY_DOWN, MOVE)
is causing the error.
If you need to access the stage, add a listener in the constructor for the ADDED_TO_STAGE event, and in the callback function you will be able to access the stage
So:
public function Circles():void {
this.addEventListener (Event.ADDED_TO_STAGE, onAddedToStage);
this.y = 175
this.x = 10
}
private function onAddedToStage (evt:Event):void {
stage.addEventListener(KeyboardEvent.KEY_DOWN, MOVE)
}