Actionscript 3 cannot access a property or method of a null object reference - actionscript-3

Question

I'm still really new about classes and stuffs. So, I tried making this and I got an error: Access of undefined property.
Why speedX and speedY var still error although I've defined it in public var in the main class?
Thanks!
EDITED: I've tried calling the variables from other class with main.speedX and main.speedY
But it got error : Cannot access a property or method of a null object reference.
at Ball/moveBall()
This is the Main code:
package
{
import flash.display.MovieClip;
import flash.events.Event;
public class Main extends MovieClip
{
public var speedX:Number = 5;
public var speedY:Number = 5;
public var speedMax:Number = 10;
private var ball:MovieClip = new Ball();
private var paddle:MovieClip = new Paddle();
public function Main()
{
paddle.addEventListener(Event.ENTER_FRAME, movePaddle);
addChild(ball);
addChild(paddle);
}
}
}
This is the Ball Movie Clip Code:
package
{
import flash.display.MovieClip;
import flash.events.Event;
public class Ball extends MovieClip
{ public var main:Main;
public function Ball()
{addEventListener(Event.ENTER_FRAME, moveBall);
main= new Main();
}
public function moveBall(e:Event):void
{
x += main.speedX;
y += main.speedY;
}
}
}

Answer 1

That's because your class Ball cannot access speedX and speedY inside the event callback. Why not add speedX and speedY to your Ball class directly instead ?
public class Ball extends MovieClip
{
public var speedX:Number;
public var speedY:Number;
public function Ball(sX:Number = 0, sY:Number = 0)
{
this.speedX = sX;
this.speedY = sY;
addEventListener(Event.ENTER_FRAME, moveBall);
}
public function moveBall(e:Event):void
{
x += speedX;
y += speedY;
}
}

Answer 2

Here's another possible solution where you would be passing main to ball to use the values of speed stored in Main.
public class Main extends MovieClip
{
public var speedX:Number = 5;
private var ball:MovieClip;
public function Main()
{
ball=new Ball(this);
addChild(ball);
}
}
and
public class Ball extends MovieClip
{
private var _main:Main;
public function Ball(main:Main)
{
_main=main;
addEventListener(Event.ENTER_FRAME, moveBall);
}
public function moveBall(e:Event):void
{
x += _main.speedX;
}
}
}