Flash錯誤＃1009：無法訪問空對象引用....的屬性或方法？

Question

我使用flash as3創建菜單時出現問題。你們能幫助我應該怎麼做...？就像這樣：

TypeError: Error #1009: Cannot access a property or method of a null object reference. 
at DocumentClass/menuScreen() 
at DocumentClass/mouseBack() 

如何正確的代碼來解決代碼如下：

package { 
import flash.display.MovieClip; 
import flash.events.Event; 
import flash.display.SimpleButton; 
import flash.events.MouseEvent; 
import flash.system.fscommand; 
import flash.display.StageDisplayState; 

public class DocumentClass extends MovieClip { 
public var backbutton:BackButton = new BackButton(); 
    public function DocumentClass() { 
     menuScreen(); 
     stage.addEventListener(Event.ENTER_FRAME, masukGame); 

    } 
    public function masukGame(e:Event):void{ 
     stage.displayState = StageDisplayState.FULL_SCREEN; 
    } 
    public function menuScreen():void{ 

     game.menuscreen.mb.addEventListener(MouseEvent.MOUSE_DOWN, mouseClick1); 
     game.menuscreen.pb.addEventListener(MouseEvent.MOUSE_DOWN, mouseClick2); 
     game.menuscreen.mlb.addEventListener(MouseEvent.MOUSE_DOWN, mouseClick3); 
     game.menuscreen.kb.addEventListener(MouseEvent.MOUSE_DOWN, mouseClick4); 


    } 
    public function backMenu():void{ 

     backbutton.buttonMode = true; 
     backbutton.addEventListener(MouseEvent.MOUSE_DOWN, mouseBack); 
     backbutton.x = 450; 
     backbutton.y = 400; 
     // Dan akhirnya letakan button ke stage; 
     addChild(backbutton); 
     backbutton.visible=true; 

    } 

    public function mouseClick1(event:MouseEvent):void { 
     game.gotoAndStop("MainGame"); 
     backMenu(); 


    } 
    public function mouseClick2(event:MouseEvent):void { 
     game.gotoAndStop("Petunjuk"); 
     backMenu(); 
    } 
    public function mouseClick3(event:MouseEvent):void { 
     game.gotoAndStop("Malaria"); 
     backMenu(); 
    } 
    public function mouseClick4(event:MouseEvent):void { 
     game.gotoAndStop("Keluar"); 
     backMenu(); 
    } 
    public function mouseBack(event:MouseEvent):void { 

     menuScreen(); 


    } 


    } 
} 

這是我的文件，在這裏下載：https://www.dropbox.com/s/jo7bcwnlfj7qjnh/My%20Game.rar

請幫助我，感謝傢伙..

Answer 1

你的問題是空指針異常。
這意味着你正試圖訪問一個空對象內的變量。

您提供的StackTrace指導我們解決問題。
跟蹤中的最後一個地方是：

DocumentClass/menuScreen() 

所以，你的問題是DocumentClass類的menuScreen()內。

public function menuScreen():void 
{ 
    game.menuscreen.mb.addEventListener(MouseEvent.MOUSE_DOWN, mouseClick1); 
    game.menuscreen.pb.addEventListener(MouseEvent.MOUSE_DOWN, mouseClick2); 
    game.menuscreen.mlb.addEventListener(MouseEvent.MOUSE_DOWN, mouseClick3); 
    game.menuscreen.kb.addEventListener(MouseEvent.MOUSE_DOWN, mouseClick4); 
} 

在這裏，我們試圖訪問：

• game
• game.menuscreen
• game.menuscreen.mb
• game.menuscreen.pb
• game.menuscreen.mlb
• game.menuscreen.kb

其中之一爲空。

---

編輯1

好吧，我想你的代碼的.fla並發現問題是game.menuscreen電話。
一旦你改變你的屏幕，你從舞臺上刪除menuscreen。

當您回到並嘗試訪問它時，它不再存在，因此，您將得到一個NPE（空指針異常）。

---

EDIT 2

你可以用不同的方法，但最簡單的（和最靈活的）解決問題是一個參考存到自己menuscreen的地方，你可以得到它後者。

例如：你可以把它放在DocumentClass類是這樣的：

public class DocumentClass extends MovieClip 
{ 
    public var backbutton:BackButton = new BackButton(); 
    public var gameMenuScreen:MovieClip; 

    public function DocumentClass() 
    { 
     gameMenuScreen = game.menuscreen; 
     menuScreen(); 
     stage.addEventListener(Event.ENTER_FRAME, masukGame); 
    } 

    // ... rest of the class 
} 

然後你可以從DocumentClass直接訪問它，像這樣：

public function menuScreen():void 
{ 
    // It was "game.menuscreen" 
    // Now it is "gameMenuScreen" 

    gameMenuScreen.mb.addEventListener(MouseEvent.MOUSE_DOWN, mouseClick1); 
    gameMenuScreen.pb.addEventListener(MouseEvent.MOUSE_DOWN, mouseClick2); 
    gameMenuScreen.mlb.addEventListener(MouseEvent.MOUSE_DOWN, mouseClick3); 
    gameMenuScreen.kb.addEventListener(MouseEvent.MOUSE_DOWN, mouseClick4); 
} 

Answer 2

的解決方案是閃光燈改變的版本另一種。我試了一下，問題就消失了。