空指针异常正交相机

Question

我正在为我的libgdx游戏创建一个菜单屏幕。如果我的菜单播放按钮被触摸，我想绘制一个矩形来检测。不幸的是，通过取消投影相机，我得到了一个Exception甚至很难我初始化相机并检查多次错误。不要怀疑OrthoCamera类它只是继承自OrthographicCamera类并具有一些额外的功能。

菜单类：

public class MenuScreen extends Screen {


private OrthoCamera cam;

private PlayButton pb;
private ExitButton eb;
private Background bg;

@Override
public void create() {
    // TODO Auto-generated method stub
    cam = new OrthoCamera();

    cam.resize():

    pb = new PlayButton();

    eb = new ExitButton();

    bg = new Background();

播放按钮类：

public class PlayButton  {

Texture texture;
Sprite sprite;

public PlayButton(){

    texture = new Texture("button.png");

    texture.setFilter(TextureFilter.Linear, TextureFilter.Linear);
    TextureRegion region = new TextureRegion(texture, 59, 52, 300,250);

    sprite = new Sprite(region);
    sprite.setSize(sprite.getWidth() / 2, sprite.getHeight() / 2);
    sprite.setOrigin(-sprite.getWidth() / 2, -sprite.getHeight() / 2);

    sprite.setPosition(MyGdxGame.WIDTH / 2  - 317, MyGdxGame.HEIGHT /2 -110);
    }

    public void render(SpriteBatch batch) {
    // TODO Auto-generated method stub
    System.out.println("es wird gerendert");
    sprite.draw(batch);
}

    public void update(OrthoCamera cam) {

        // Source: http://stackoverflow.com/questions/24834399/how-to-detect-if-a-texture-has-been-touched-libgdx-without-scene2d

        System.out.println("update");
         if(Gdx.input.isTouched())
         {
       Vector3 tmp = new Vector3(Gdx.input.getX(),Gdx.input.getY(),0);
       cam.unproject(tmp);
      Rectangle textureBounds=new Rectangle((MyGdxGame.WIDTH / 2-317),   
        (MyGdxGame.HEIGHT /2 -110), 300, 250 );


          if(textureBounds.contains(tmp.x,tmp.y))
              {
               ScreenManager.setScreen(new GameScreen());
              }
         } 

         }



}

@Override
public void render(SpriteBatch batch) {

    batch.begin();
    bg.render(batch);
    pb.render(batch);
    eb.render(batch);

    batch.end();

}


 public void update(OrthoCamera cam) {
    // TODO Auto-generated method stub
    //cam.update();
    pb.update(cam);
}

如果我点击屏幕，游戏崩溃，控制台说：

       Exception in thread "LWJGL Application" java.lang.NullPointerException
at buttons.PlayButton.update(PlayButton.java:58)
at screen.MenuScreen.update(MenuScreen.java:109)