我得到一个空指针错误，我不知道为什么？

Question

我正在尝试使其能够使用EventHandlers来移动角色，但是在这一部分中遇到了此空指针错误，并且我不知道如何解决。如果有人对如何解决有任何想法，将不胜感激。我收到错误

  

由以下原因引起：myFirstGame.honeyBee.start（honeyBee.java:69）上的java.lang.NullPointerException

这是这行代码：scene.addEventHandler(KeyEvent.KEY_PRESSED, (key) -> {

是因为scene.add吗？我不太确定从哪里开始

public class honeyBee extends Application {

    GameObject gameObject;
    GraphicsContext gc;
    Stage stage;
    Canvas canvas;
    Image img1;
    Factory Factory;
    Scene scene;
    private BeeCharacter bee;

    ArrayList<GameObject>characterList = new ArrayList<GameObject>();

    ArrayList<GameObject>Hive = new ArrayList<GameObject>();

    ArrayList<GameObject>flowers = new ArrayList<GameObject>();

    Random rnd = new Random(System.currentTimeMillis());
    int count = 0;

    AnimationTimer timer = new AnimationTimer() {

        @Override
        public void handle(long now) {
            // TODO Auto-generated method stub
            gc.drawImage(img1, 0, 0, canvas.getWidth(), canvas.getHeight());
            for(GameObject obj : characterList) {
                obj.update();
            }

            for(GameObject obj : Hive) {
                obj.update();
            }

            if(count++>40) {
                flowers.add(new SpawnFlowers(gc, rnd.nextInt(600), rnd.nextInt(550)));
                count = 2;
            }
            for(GameObject obj : flowers) 
                ((SpawnFlowers)obj).grow();
        }
    };

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        launch(args);
    }

    public void start(Stage stage) throws Exception {
        stage.setTitle("HoneyBee");
        Pane root = new Pane();
        //scene = new Scene(root, canvas.getWidth(), canvas.getHeight());
        scene.addEventHandler(KeyEvent.KEY_PRESSED, (key) -> {
            if(key.getCode()==KeyCode.UP) {
                  bee.setSpeedY(5);
                  bee.setDirectionY(-1);
              } else if(key.getCode()==KeyCode.LEFT) {
                  bee.setSpeedX(5);
                  bee.setDirectionX(-1);
              } else if(key.getCode()==KeyCode.DOWN) {
                  bee.setSpeedY(5);
                  bee.setDirectionY(1);
              } else if(key.getCode()==KeyCode.RIGHT) {
                  bee.setSpeedX(5);
                  bee.setDirectionX(1);
              }

      });

        scene.addEventHandler(KeyEvent.KEY_RELEASED, (key) -> {           
              if(key.getCode()==KeyCode.UP) {
                  bee.setSpeedY(0);
                  bee.setDirectionY(0);
              } else if(key.getCode()==KeyCode.LEFT) {
                  bee.setSpeedX(0);
                  bee.setDirectionX(0);
              } else if(key.getCode()==KeyCode.DOWN) {
                  bee.setSpeedY(0);
                  bee.setDirectionY(0);
              } else if(key.getCode()==KeyCode.RIGHT) {
                  bee.setSpeedX(0);
                  bee.setDirectionX(0);
              }  
        });

        stage.setScene(new Scene(root, 780, 580));
        stage.setResizable(false);
        stage.show();   

        canvas = new Canvas(800,600);
        gc = canvas.getGraphicsContext2D();
        Factory = new Factory(gc);
        img1 =new Image("res/1.png");
        gc.drawImage(img1, 0, 0, canvas.getWidth(), canvas.getHeight());
        root.getChildren().add(canvas);

        bee = (BeeCharacter)Factory.createCharacter("bee", 400, 300, 0, 0);
        Hive.add(Factory.createCharacter("Hive", 700, 300, 0, 0));  

        timer.start();
    }


}

我的蜜蜂班

class BeeCharacter extends GameObject{

    protected double speedX, speedY=0;
    protected double dx, dy=0;
    protected int score=0;


    public BeeCharacter(GraphicsContext gc, double x, double y) {
        super(gc, x, y);
        img = new Image("res/bee.png");
        update();
    }

    public void update() {
        if(x+speedX*dx>gc.getCanvas().getWidth() || x+speedX*dx < 0)
        {
            setDirectionX(0);
        } else {
            x+=speedX*dx;
        }

        if(y+speedY*dy>gc.getCanvas().getHeight() || y+speedY*dy < 0)
        {
            setDirectionY(0);
        } else {
            y+=speedY*dy;
        }
        super.update();
    }

    public void setSpeedY(double speedY) {
        this.speedY = speedY;
    }

    public void setSpeedX(double speedX) {
        this.speedX = speedX;
    }

    public void setDirectionX(int dir) {
        dx = dir;
    }
    public void setDirectionY(int dir) {
        dy = dir;
    }

    public void setLocation(int x, int y)
    {
        this.x = x;
        this.y = y;
    }

}