在Vector中插入元素时,我收到错误java.lang.NullPointerException

当我尝试运行以下程序时,它会抛出 java.lang.NullPointerException 。我不明白,为什么在将元素插入向量时会出现此错误。请帮助我解决以下问题。提前感谢。

    public class GameMidlet extends MIDlet implements CommandListener {

    GameCanvas game;
    Display display;
    Command exitCommand;
    Command restartCommand;
    Vector mGameCanvasList;

    public void startApp() {

        display  = Display.getDisplay(this);
        game = new GameCanvas();

        //i am getting error at here when game object inset in vector
        mGameCanvasList.addElement(game);

        GameCanvas fistList = (GameCanvas) mGameCanvasList.elementAt(0);
        display.setCurrent(fistList);

        exitCommand     = new Command("Exit", Command.EXIT, 0);
        restartCommand  = new Command("Restart", Command.OK, 0);

        fistList.addCommand(exitCommand);
        fistList.addCommand(restartCommand);

        fistList.setCommandListener(this);
        fistList.setCommandListener(this);
        fistList.startThread();

    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
    }

    public void commandAction(Command c, Displayable d) {

        if(c == exitCommand){
//            System.out.println("Exit");

            destroyApp(true);
            notifyDestroyed();
        }

        if(c == restartCommand){
//            game
        }

    }
}