The 1009 error on Loader.load()

Just a note about 1009 error on Loader.load()
the situation is that the main.swf attempts to load child.swf

on Child class

package {

 import flash.events.Event;
 import flash.display.Sprite;

 public class Child extends Sprite {

  public function Child() {
   initChild();
  }
  
  public function initChild():void {
   stage.focus = this;
  }
 }
}

on Main class

package net.littleshell {
 import flash.display.Loader;
 import flash.display.Sprite;
 import flash.events.Event;
 import flash.net.URLRequest;
 
 public class Main extends Sprite {
  
  private var l:Loader
  
  public function Main() {
   init();
  }
  
  public function loadSWF($url:String):void {
   var rq:URLRequest = new URLRequest($url);
   l.load(rq);
  }
  
  private function init():void {
   initLoader();  
   loadSWF("child.swf");
  }
  
  private function initLoader():void {
   l = new Loader();
   l.contentLoaderInfo.addEventListener(Event.COMPLETE, loader_complete);
   
   addChild(l);
  }
  
  private function loader_complete(e:Event):void {
   //blar blar blar
  }
 }
}

in such case, a #1009 error will be thrown in child.swf it is because child.swf references the stage object too soon.
to solve it, simply run your code when ADDED_TO_STAGE event was dispatched.

Modified Child class

package net.littleshell {

 import flash.events.Event;
 import flash.display.Sprite;

 public class Child extends Sprite {

  public function Child() {
   addEventListener(Event.ADDED_TO_STAGE, initChild);
   initChild();
  }
  
  public function initChild(e:Event == null):void {
   removeEventListener(Event.ADDED_TO_STAGE, initChild)
   
   //its now safe to do anything
   stage.focus = this;
  }
 }
}