当我在JAVAFX中更改场景时出现NullPointerException

当我按下startgame按钮和exitgame按钮时，我想改变我的舞台上的场景或关闭舞台，但它只是向我展示第一个场景，当我试图在NullPointerException的情况下将其中一个按钮编译器关闭时 .

我也有主类和两个fxml文件，但我认为没有必要把它放在这里，它只包含两个锚窗格，原始标签和按钮 .

package sample;

import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.fxml.Initializable;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.AnchorPane;
import javafx.stage.Stage;

import java.io.IOException;
import java.net.URL;
import java.util.ResourceBundle;

public class Controller implements Initializable{

    private Stage stage = new Stage();

    private FXMLLoader loader = new FXMLLoader();

    private AnchorPane anchorPane = new AnchorPane();

    private Scene scene = new Scene(new AnchorPane());

    @FXML private Button startGameButton;
    @FXML private Button endGameButton;

    @FXML private Button buttonAnswer1;
    @FXML private Button buttonAnswer2;
    @FXML private Button buttonAnswer3;
    @FXML private Button buttonAnswer4;

    public void createScene(int typeOfScene) throws Exception
    {
        if (typeOfScene == 1)
        {
            loader = new FXMLLoader(getClass().getResource("/sample/sample.fxml"));
            anchorPane = loader.load();

            scene = new Scene(anchorPane);
            stage.setScene(scene);
            stage.show();
        }

        if (typeOfScene == 2)
        {
            scene = null;

            loader = new FXMLLoader(getClass().getResource("/sample/episodesFXML.fxml"));

            anchorPane = loader.load();

            scene = new Scene(anchorPane);
            stage.setScene(scene);
            stage.show();
        }
    }

    public void getPrimaryStage(Stage stage) throws Exception
    {
        this.stage = stage;
        createScene(1);
    }

    @Override
    public void initialize(URL location, ResourceBundle resources) {
        startGameButton.setOnAction(event -> {
            try {
                createScene(2);
            } catch (Exception e)
            {
                e.printStackTrace();
            }
        });

        endGameButton.setOnAction(event -> {
            stage.close();
        });
    }
}