getChildByName loop inside mc - actionscript-3

Question

I'm trying to get the getChildByName work by loop inside movieclip.
On library i have a movieclip called PlayScreen, inside PlayScreen i have another movieclip called Bg. PlayScreen is put on stage by MainClass (class document) and PlayScreen is link to MainRun class. Bg movieclip is manually added to the PlayScreen by dragging it in from the library.
here is my MainRun class.
public class MainRun extends MovieClip
{
public var flyClone:Array;
public var newFly:Fly_MC;
public var spX:Number = 550;
public var spY:Number = 400;
public function MainRun()
{
flyClone = new Array();
for(var i:int=0; i<10; i++)
{
newFly = new Fly_MC(Math.random()* spX, Math.random()* spY);
flyClone.push(newFly);
addChild(newFly);
newFly.name = "fly_" + i;
}
//...
bla();
}
and here i couldn't make the getChildByName work properly, and it return; Error #1009: Cannot access a property or method of a null object reference.
at MainRun/bla()
public function bla():void
{
for(var j:int=0; j< numChildren; j++)
{
getChildByName("fly_" + j).addEventListener(Event.ENTER_FRAME, randomMove);
//...
}
}
any suggestion?

Answer 1

name property of your MovieClip object must is a String value, but you are putting an int there.
Try adding String(j);
It's better to use your flyClone array instead of just getting objects by it's name.
Image what will happen if you add anything else to the DisplayList (addChild).
for(var j:int=0; j< flyClone.length; j++)
{
flyClone[j].addEventListener(...);
}

Answer 2

I am almost positive you have a child that is not a fly.
public function bla():void
{
for(var j:int=0; j< numChildren; j++)
{
var fly : Fly_MC = getChildByName("fly_" + j) as Fly_MC;
if( fly )
{
fly.addEventListener(Event.ENTER_FRAME, randomMove);
}
}
}
Why don't you just:
for each( var fly : Fly_MC in flyClone )
{
fly.addEventListener(Event.ENTER_FRAME, randomMove);
}
or better
addEventListener(Event.ENTER_FRAME, randomMoveFlies);
public function randomMoveFlies( e:Event ):void
{
for each( var fly : Fly_MC in flyClone )
{
//move flies;
}
}

Answer 3

for(var j:int=0; j< flyClone.length; j++)
{
(flyClone[j] as MovieClip).addEventListener(Event.ENTER_FRAME, randomMove);
}