Ich versuche, einige Werte aus der MySQL-Datenbank zu lesen und sie in einem Menü in JavaFX anzuzeigen. Mein Code ist unten. Früher habe ich Buttons anstelle von Menu verwendet. Es hat gut funktioniert, aber ich habe mich mit Buttons nicht besser gefühlt, also habe ich mich für Menu entschieden. Ich habe die in der Überschrift erwähnte Ausnahme erhalten, als ich diesen Code ausgeführt habe.

private VBox userSelection() throws ClassNotFoundException, SQLException {

    VBox vb1 = new VBox();
    vb1.setPadding(new Insets(40, 150, 20, 200));
    vb1.setSpacing(20);

    MenuBar menuBar = new MenuBar();
    Menu menuFile1 = new Menu("CHOOSE YOUR ACCOUNT");
    menuFile1.setStyle("-fx-border-color: green; -fx-font-size: 14pt; "
            + "-fx-font-family: Comic Sans MS; -fx-padding: 1 10 1 1");

    Text scenetitle2 = new Text("Choose Your Account");
    scenetitle2.setFont(Font.font("Tahoma", FontWeight.BOLD, 20));
    scenetitle2.setUnderline(true);
    vb1.getChildren().addAll(scenetitle2);

    Class.forName("com.mysql.jdbc.Driver");
    connect = DriverManager
            .getConnection("jdbc:mysql://localhost:3306/project?"
                    + "user=root&password=virus");
    statement = connect.createStatement();

    rs = statement.executeQuery("select * from user");

    while (rs.next()) {

        String username = rs.getString("staffname");

        MenuItem add = new MenuItem(username);
        add.setStyle(" -fx-font-size: 18pt; "
            + "-fx-font-family: Calibri; -fx-padding: 1 10 1 1");
        menuFile1.getItems().addAll(add);
        menuBar.getMenus().addAll(menuFile1);

        vb1.getChildren().addAll(menuBar);
    }

    return vb1;
}

Die Ausnahmedetails sind:

java.lang.IllegalArgumentException: Children: duplicate children added: parent = VBox@872be7
at javafx.scene.Parent$1.onProposedChange(Parent.java:307)
at com.sun.javafx.collections.VetoableObservableList.addAll(VetoableObservableList.java:106)
at com.sun.javafx.collections.ObservableListWrapper.addAll(ObservableListWrapper.java:160)
at com.sun.javafx.collections.ObservableListWrapper.addAll(ObservableListWrapper.java:309)
at frontpage.FrontPage.userSelection(FrontPage.java:466)
at frontpage.FrontPage.access$100(FrontPage.java:44)
at frontpage.FrontPage$2.handle(FrontPage.java:129)
at frontpage.FrontPage$2.handle(FrontPage.java:125)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:28)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.Node.fireEvent(Node.java:6867)
at javafx.scene.control.Button.fire(Button.java:179)
at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:193)
at com.sun.javafx.scene.control.skin.SkinBase$4.handle(SkinBase.java:336)
at com.sun.javafx.scene.control.skin.SkinBase$4.handle(SkinBase.java:329)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:64)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3311)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3151)
at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3106)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1563)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2248)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:250)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:173)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:292)
at com.sun.glass.ui.View.handleMouseEvent(View.java:530)
at com.sun.glass.ui.View.notifyMouse(View.java:924)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$100(WinApplication.java:17)
at com.sun.glass.ui.win.WinApplicationIst der Name des Personals eindeutig?1.run(WinApplication.java:67)
at java.lang.Thread.run(Thread.java:744)
Exception in thread "JavaFX Application Thread" 

Wie kam es zu dieser Ausnahme? Wie kann ich das korrigieren?