Making a scrolling shooter, can't figure out glitch!

Okay, so I've got a pretty simple scrolling shooter going on, I started working on it in a CS3 class I was taking and at the very end of the course I ran into this problem and haven't been able to figure it out on my own! I know exactly what the error is, but I have absolutely no idea what's causing it or why, and it happens completely randomly. Sometimes I can play the game for 10 minutes and it'll work perfectly, sometimes I can play it for 10 seconds and then it will glitch and freak out. :crazy: I'd like to try and explain it here more, but I feel as if it would be much easier for you to just run the game and see it for yourself.

I've attached the .fla, I hope someone here can figure out what's going on! I apologize if my code is rather messy, as it should be obvious I'm still a Flash and AS3 noob and my formating habits probably aren't up to standard.

Here's a slightly updated version, I do not understand how I can be getting this error now.

The error is when the function tries to reference a null object in the moveEnemy script, but here is the moveEnemy script:

Code:

---

function moveEnemy():void
{
        var enemyMove:Number;
        for(enemyMove=0; enemyMove<enemies_arr.length; enemyMove++)
        {
                if(enemies_arr[enemyMove]!=null)
                {
                        //if(enemies_arr[enemyMove].y<900)
                        //{
                        enemies_arr[enemyMove].y+=enemySpeed;
                        //enemies_arr[enemyMove].x+=enemyXVelocity();
                        //}
                        if(enemies_arr[enemyMove].y==900 && enemies_arr[enemyMove].enemyShipBasic_ani.alpha==1)
                        {
                                playerHealth+=10;
                                killEnemy(enemyMove);
                                //enemies_arr[enemyMove].enemyShipBasic_ani.alpha=0;
                        }
                }
        }
}

---

Now, please tell me, how could it possibly be trying to reference a null object when it's checking if the object is null before continuing?