添加一个按键侦听器的动作脚本3

我是比较新的动作脚本，我试图做一个游戏蛇。很显然，我需要实现一个全球性的按键侦听器，但我有奇怪的问题。我尝试添加监听器应用程序标记，但它似乎没有任何效果（电影依然能编译）。每当我称之为

I am relatively new to Action Script, and I am trying to make a game of Snake. Obviously I need to implement a global key listener, but I am having odd problems. I tried adding the listener to the application tag but it didn't seem to have any effect (the movie was still able to compile). Whenever I call

this.stage.addEventListener（KeyboardEvent.KEY_DOWN，钥匙，真正的）;

我的程序崩溃。下面是我的main.mxml文件的内容：

my program crashes. Below is the content of my main.mxml file:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application 
 xmlns:mx="http://www.adobe.com/2006/mxml" 
 layout="absolute"
 width="800" 
 height="600"
 frameRate="15"
 creationComplete="creationComplete();"
 enterFrame="enterFrame(event);"
 currentState="MainMenu">

 <mx:states>
  <mx:State 
   name="Game"   
   enterState="enterGame(event)"
         exitState="exitGame(event)">
  </mx:State>
  <mx:State 
   name="LevelEnd">
   <mx:AddChild relativeTo="{myCanvas}" position="lastChild">
    <mx:Button x="380" y="344" label="Continue" id="btnContinue" click="btnContinueClicked(event)" width="90" height="30"/>
   </mx:AddChild>
   <mx:AddChild relativeTo="{myCanvas}" position="lastChild">
    <mx:Label x="10" y="10" text="Congratulations, you finished the level."/>
   </mx:AddChild>
  </mx:State>
  <mx:State name="MainMenu">
   <mx:AddChild relativeTo="{myCanvas}" position="lastChild">
    <mx:Button x="381" y="344" label="Start" id="btnStart" click="startGameClicked(event)" width="90" height="30"/>
   </mx:AddChild>
   <mx:AddChild relativeTo="{myCanvas}" position="lastChild">
    <mx:Image x="10" y="49" source="@Embed('../media/mainmenu.png')"/>
   </mx:AddChild>
   <mx:AddChild relativeTo="{myCanvas}" position="lastChild">
    <mx:Label x="10" y="10" text="Snake Pro" fontSize="20" fontWeight="bold"/>
   </mx:AddChild>
  </mx:State>
 </mx:states>

 <mx:Canvas x="0" y="0" width="100%" height="100%" id="myCanvas"/>

 <mx:Script>
 <![CDATA[   

  protected var inGame:Boolean = false;
  protected var currentLevel:int = 1;
  import flash.events.KeyboardEvent; 

  public function creationComplete():void
     {
   LevelDefinitions.Instance.startup();
   addKeyEvent();
   //stage.focus = stage;
     }

  private function addKeyEvent():void
  {
   this.stage.addEventListener(KeyboardEvent.KEY_DOWN, key, true);
  }

     public function enterFrame(event:Event):void
     {
      if (inGame)
      {
       GameObjectManager.Instance.enterFrame();

       myCanvas.graphics.clear();
       myCanvas.graphics.beginBitmapFill(GameObjectManager.Instance.backBuffer, null, false, false);
       myCanvas.graphics.drawRect(0, 0, this.width, this.height);
       myCanvas.graphics.endFill();
      } 
     }

        private function key(event:KeyboardEvent):void {
            //t1.text = event.keyCode + "/" + event.charCode;

   GameObjectManager.Instance.setDirection(0, 1);
   currentState = "MainMenu";
   inGame = false;
        }

     protected function startGameClicked(event:Event):void
  {
   currentState = "Game"
  }     

     protected function enterGame(event:Event):void
  {
   Mouse.hide();
   GameObjectManager.Instance.startup();
   Level.Instance.startup(currentLevel);
   inGame = true;
     }

     protected function exitGame(event:Event):void
  {
   Mouse.show();
   Level.Instance.shutdown();
   GameObjectManager.Instance.shutdown();
   inGame = false;
  }  

  protected function btnContinueClicked(event:Event):void
  {
   currentLevel = LevelDefinitions.Instance.getNextLevelID(currentLevel);
   if (currentLevel == 0)
   {
    currentLevel = 1;
    currentState = "MainMenu";
   }
   else
   {
    currentState = "Game"
   }
     }
 ]]>
    </mx:Script>

</mx:Application>

此外，看来我得到这个堆栈跟踪：

Also, it seems I am getting this stack trace:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at main/addKeyEvent()[C:\Users\Me\Desktop\Flash\Snake\src\main.mxml:58]
    at main/creationComplete()[C:\Users\Me\Desktop\Flash\Snake\src\main.mxml:52]
    at main/___main_Application1_creationComplete()[C:\Users\Me\Desktop\Flash\Snake\src\main.mxml:10]
    at flash.events::EventDispatcher/dispatchEventFunction()
    at flash.events::EventDispatcher/dispatchEvent()
    at mx.core::UIComponent/dispatchEvent()[E:\dev\4.x\frameworks\projects\framework\src\mx\core\UIComponent.as:12528]
    at mx.core::UIComponent/set initialized()[E:\dev\4.x\frameworks\projects\framework\src\mx\core\UIComponent.as:1627]
    at mx.managers::LayoutManager/doPhasedInstantiation()[E:\dev\4.x\frameworks\projects\framework\src\mx\managers\LayoutManager.as:759]
    at mx.managers::LayoutManager/doPhasedInstantiationCallback()[E:\dev\4.x\frameworks\projects\framework\src\mx\managers\LayoutManager.as:1072]

我在我束手无策这里，我AP preciate你的时间和努力。谢谢！

I am at my wit's end here, and I appreciate your time and efforts. Thanks!