javafx应用程序启动时无法进行对话显示

Question

我正在尝试在用户首次运行应用程序时显示一个对话框。 我已将setOnShowing设置为调用启动对话框的showFirstRunDialog方法。当我运行应用程序时，我必须测试变量，如果它是第一次运行是真的，我得到一个例外。

错误：

Exception in Application start method
[Debug] Writing files
[Debug] writeLogs is set to false, debug logs will not be written
java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:473)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:372)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:945)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:973)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:198)
    at java.base/java.lang.Thread.run(Thread.java:844)
Caused by: java.lang.NullPointerException
    at Application.Main$1.handle(Main.java:34)
    at Application.Main$1.handle(Main.java:28)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
    at javafx.graphics/javafx.stage.Window.fireEvent(Window.java:1380)
    at javafx.graphics/javafx.stage.Window$12.invalidated(Window.java:1070)
    at javafx.base/javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:110)
    at javafx.base/javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:145)
    at javafx.graphics/javafx.stage.Window.setShowing(Window.java:1186)
    at javafx.graphics/javafx.stage.Window.show(Window.java:1201)
    at javafx.graphics/javafx.stage.Stage.show(Stage.java:276)
    at Application.Main.start(Main.java:37)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:919)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$11(PlatformImpl.java:449)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$9(PlatformImpl.java:418)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:417)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:175)
    ... 1 more
Exception running application Application.Main

这是我的主要方法：

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{
        Parent root;
        System.out.println(Settings.getInstance().isFirstRun());
        root = FXMLLoader.load(getClass().getResource("mainWindow.fxml"));
        primaryStage.setTitle("Volunteer Logger");
        primaryStage.setScene(new Scene(root, 300, 275));
        primaryStage.setOnShowing(new EventHandler<WindowEvent>() {
            @Override
            public void handle(WindowEvent event) {
                if (Settings.getInstance().isFirstRun()) {
                    MainWindowController controller;
                    FXMLLoader loader = new FXMLLoader();
                    controller = loader.getController();
                    controller.showFirstRunDialog();
                }
            }
        });
        primaryStage.show();
    }



    @Override
    public void init() throws Exception {
        //Settings.getInstance().setWriteLogs(true);
        super.init();
        Data.getInstance().loadFiles();
    }

    @Override
    public void stop() throws Exception {
        super.stop();
        Data.getInstance().writeFiles();
    }

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

谢谢！