使用带有ToggleButton的setOnAction时收到错误

Question

我试图用Java编写一个简单的音乐剧。我尝试使用切换按钮进行音乐播放或暂停，具体取决于播放器当前是否正在播放。这是我的代码现在的样子：

public class Client extends Application {
private boolean isPlaying = false;

public static void main(String[] args) {
    launch(args);
}

public void start(Stage primaryStage) throws Exception {
    Parent root = FXMLLoader.load(getClass().getResource("clientUI.fxml"));

    ToggleButton toggleButton = (ToggleButton) root.lookup("toggleButton");
    File song = new File("Shed.mp3");
    Media media = new Media(song.toURI().toURL().toExternalForm());
    MediaPlayer mp = new MediaPlayer(media);

    toggleButton.setOnAction(new EventHandler<ActionEvent>() {
        @Override
        public void handle(ActionEvent e) {
            if (isPlaying()) {
                mp.pause();
            } else {
                mp.play();
            }
        }
    });

    Scene s = new Scene(root);
    primaryStage.setScene(s);
    primaryStage.setResizable(false);
    primaryStage.setTitle("Noisey");
    primaryStage.show();

}

public boolean isPlaying() {
    return isPlaying;
}

} 但是，在运行时，会收到此错误： `

Exception in Application start method
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.launchApplication(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.launcher.LauncherHelper$FXHelper.main(Unknown Source)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$155(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
    at application.Client.start(Client.java:32)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$162(Unknown Source)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$175(Unknown Source)
    at com.sun.javafx.application.PlatformImpl.lambda$null$173(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$174(Unknown Source)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(Unknown Source)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$148(Unknown Source)
    ... 1 more
Exception running application application.Client

有些东西告诉我这一行Caused by: java.lang.NullPointerException at application.Client.start(Client.java:32)是问题所在。