Flex:how to solve Error #1009: Cannot access a property or method of a null object reference

Question

May I know how to solve the error that occur when need to import .as file into main.mxml file ? the .as file is using stagevideo function, the output is display in AIR window. May I know which part I miss up? Thank you and I appreciate if there are anyone able to help me. Thank you

    <fx:Script>
    <![CDATA[
        import flash.display.StageDisplayState;
        import flash.events.KeyboardEvent;
        import flash.ui.Keyboard;

        import mx.events.FlexEvent;

        import Video.SimpleVid;

        public var simpleVid:SimpleVid;

        protected function init():void
        {       
            simpleVid = new SimpleVid();
            addElement(simpleVid);
            stage.addEventListener(KeyboardEvent.KEY_DOWN,onKeyDown);
        }

        public function onKeyDown(event:KeyboardEvent):void
        {
            if(event.keyCode == Keyboard.O)
            {
                if(simpleVid.available)
                    simpleVid.toggleStageVideo(simpleVid.inited=!simpleVid.inited);
            }
            else if ( event.keyCode == Keyboard.F )
            {
                simpleVid.stage.displayState = StageDisplayState.FULL_SCREEN_INTERACTIVE;
            } 
            else if ( event.keyCode == Keyboard.SPACE )
            {
                simpleVid.ns.togglePause();
            }
            else if(event.keyCode == Keyboard.H)
            {
                var  matrix:Matrix =simpleVid.stage.transform.matrix;

                matrix.scale(-1,1);

                matrix.translate(stage.width,0);

                simpleVid.stage.transform.matrix=matrix;        
            }   
            else if(event.keyCode == Keyboard.R)
            {
                simpleVid.ns.resume();

            }

        }

    ]]>
</fx:Script>

The error show that the stage.addEventListener(keyboardEvent.KEY_DOWN, onKeyDown); The simpleVid is the name of simpleVid.as file, that I import into the main.mxml file.

<s:WindowedApplication xmlns:fx="http://ns.adobe.com/mxml/2009" 
                   xmlns:s="library://ns.adobe.com/flex/spark" 
                   xmlns:mx="library://ns.adobe.com/flex/mx"
                   xmlns:Video = "Video.*"
                   backgroundAlpha="0"
                   creationComplete="init()"
                   backgroundColor="#000000">

Answer 1

You can get stage by using systemManager.stage.addEventListener(KeyboardEvent.KEY_DOWN,onKeyDown);