AS3 - Why am I getting this 1009 error? (Cannot access a property or method of a null object) - actionscript-3

Question

I can't get this program to work. I Always get this error: Error #1009: Cannot access a property or method of a null object reference.
I don't understand why and would appreciate some help.
Here's my code:
Main class:
package
{
import flash.display.Sprite;
import flash.events.Event;
public class Main extends Sprite
{
private var enginge:Engine = new Engine(stage);
private var enemy:Enemy = new Enemy(100, 100);
public function Main():void
{
addChild(enemy);
}
}
}
Engine class:
package
{
import flash.display.Sprite;
import flash.display.Stage;
public class Engine
{
public static var stage:Stage;
public static var gravity:int = 1;
public function Engine(stage:Stage)
{
Engine.stage = stage;
}
public static function gravitate(object:Sprite):void
{
object.y += Engine.gravity;
if (object.y < Engine.stage.stageHeight - object.height / 2)
{
Engine.gravity += 1;
}
else
{
Engine.gravity = 0;
object.y = Engine.stage.stageHeight - object.height / 2;
}
}
}
}
Enemy class:
package
{
import flash.display.Sprite;
import flash.display.Stage;
import flash.events.Event;
public class Enemy extends Sprite
{
private var gravity:int = 1;
public function Enemy(x:int, y:int)
{
this.graphics.beginFill(1, 1);
this.graphics.drawRect(this.x - 25, this.y - 40, 50, 80);
this.graphics.endFill();
this.x = x;
this.y = y;
this.addEventListener(Event.ENTER_FRAME, function(e:Event):void
{
Engine.gravitate(this);
});
}
}
}

Answer 1

In order to fix the problem you need to get rid of the anonymous function in the Enemy class.
You will have:
public function Enemy(x:int, y:int)
{
this.graphics.beginFill(1, 1);
this.graphics.drawRect(this.x - 25, this.y - 40, 50, 80);
this.graphics.endFill();
this.x = x;
this.y = y;
this.addEventListener(Event.ENTER_FRAME,handler);
}
private function handler(event:Event):void
{
Engine.gravitate(Sprite(this));
}
and the code will be working. This is due to context difference of word this inside anonymous function.
Usage of anonymous function is terrible practice and you should refrain from doing it.