Cannot access a property or method of a null object reference - apache-flex

Question

I try to do preloder in Flex for my project written in Flash.
I make this with the help of this site
link text
My Flash project have next source in main class called Game
stage.addEventListener(KeyboardEvent.KEY_DOWN, keyDown);
stage.addEventListener(KeyboardEvent.KEY_UP, keyUp);
private function keyDown(event:KeyboardEvent) {
if (event.keyCode == 81 && q_was_push == false) q_was_push = true;
if (event.keyCode == 81) press_q = true;
if (event.keyCode == 65) press_a = true;
if (event.keyCode == 83) press_s = true;
if (event.keyCode == 32) press_space = true;
} ...
When I take new swf file maked by Flex, I have error
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Game()
if I comment
//stage.addEventListener(KeyboardEvent.KEY_DOWN, keyDown);
//stage.addEventListener(KeyboardEvent.KEY_UP, keyUp);
Flex application work but Flash application does not react to button presses
Please how I can make preloader and work buttons together

Answer 1

The stage property will be null until a display object is added to the display list. Listen to the addedToStage event and add the key listeners from there.
addEventListener(Event.ADDED_TO_STAGE, onAddedToStage);
function onAddedToStage(e:Event):void
{
stage.addEventListener(KeyboardEvent.KEY_DOWN, keyDown);
stage.addEventListener(KeyboardEvent.KEY_UP, keyUp);
}

Answer 2

Anytime you need access to the stage, have the Class listen for it/check for it in the constructor, and have your init function be the handler.
package
{
import flash.display.Sprite;
import flash.events.Event;
/**
* ...
* #author Brian Hodge
*/
public class SomeClass extends Sprite
{
public function SomeClass()
{
if (stage) _init();
else addEventListener(Event.ADDED_TO_STAGE, _init);
}
private function _init(e:Event = null):void
{
//You may now access the stage property of the DisplayObject.
stage.addEventListener(Event.RESIZE);
}
}
}