Run code:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuBar;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class JavaApplication39 extends Application {

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

    @Override
    public void start(Stage stage) throws Exception {
        MenuBar mb = new MenuBar();
        for (int i = 0; i< 5; i++) {
            Menu menu = new Menu(String.valueOf(i));
            mb.getMenus().add(menu);
        }
        
        VBox vb = new VBox();
        vb.getChildren().add(mb);
        
        Scene scene = new Scene(vb, 300, 300);
        stage.setScene(scene);
        stage.show();
        
        mb.getMenus().clear();
    }
}

Exception:
Exception in Application start method
Exception in thread "main" java.lang.RuntimeException: Exception in Application start method
at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:403)
at com.sun.javafx.application.LauncherImpl.access$000(LauncherImpl.java:47)
at com.sun.javafx.application.LauncherImpl$1.run(LauncherImpl.java:115)
at java.lang.Thread.run(Thread.java:722)
Caused by: java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.rangeCheck(ArrayList.java:604)
at java.util.ArrayList.get(ArrayList.java:382)
at com.sun.javafx.collections.ObservableListWrapper.get(ObservableListWrapper.java:208)
at com.sun.javafx.scene.control.skin.MenuBarSkin.rebuildUI(MenuBarSkin.java:400)
at com.sun.javafx.scene.control.skin.MenuBarSkin.access$1100(MenuBarSkin.java:76)
at com.sun.javafx.scene.control.skin.MenuBarSkin$6.onChanged(MenuBarSkin.java:238)
at com.sun.javafx.collections.ListListenerHelper$SingleChange.fireValueChangedEvent(ListListenerHelper.java:134)
at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:48)
at com.sun.javafx.collections.ObservableListWrapper.callObservers(ObservableListWrapper.java:97)
at com.sun.javafx.collections.ObservableListWrapper.clear(ObservableListWrapper.java:184)
at javaapplication39.JavaApplication39.start(JavaApplication39.java:31) <---------------- Point in application - call clear().
at com.sun.javafx.application.LauncherImpl$5.run(LauncherImpl.java:319)
at com.sun.javafx.application.PlatformImpl$4.run(PlatformImpl.java:175)
at com.sun.javafx.application.PlatformImpl$3.run(PlatformImpl.java:142)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$100(WinApplication.java:29)
at com.sun.glass.ui.win.WinApplication$2$1.run(WinApplication.java:62)
... 1 more
Java Result: 1