问将HBox放在BorderPane中会引发start方法JavaFX中的异常

EN

 public class Main extends Application {

        Stage window;
        TextField nameInput, quantityInput;
        TreeView<String> tree;

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

        public void start (Stage primaryStage) throws Exception{

            window = primaryStage;
            window.setTitle("GrocerieList");

            // tree view
            TreeItem<String> root, food, drink;

            root = new TreeItem<>();
            root.setExpanded(true);

            // food
            food = makeBranch("Food",root);
            makeBranch("Banana", food);
            makeBranch("Coconut", food);
            makeBranch("Eggs", food);

            // drink
            drink = makeBranch("Drinks",root);
            makeBranch("Water", drink);
            makeBranch("Fanta", drink);
            makeBranch("Beer", drink);

            // tree
            tree = new TreeView<>(root);
            tree.setShowRoot(false);

            // bottom input fields
            nameInput = new TextField();
            nameInput.setPromptText("Productname");
            nameInput.setMinWidth(100);

            quantityInput = new TextField();
            quantityInput.setPromptText("Quantity");
            quantityInput.setMinWidth(100);

            // buttons
            Button addbutton = new Button("Add");

            // Layout
            // HBOX
            HBox hBox = new HBox();
            hBox.setPadding(new Insets(10));
            hBox.setSpacing(10);
            hBox.getChildren().addAll(nameInput, quantityInput, addbutton);

            // BorderPane
            BorderPane borderPane = new BorderPane();
            borderPane.getChildren().addAll(tree, hBox);
            borderPane.setCenter(tree);
            borderPane.setBottom(hBox);

            Scene scene = new Scene(borderPane, 520, 620);
            window.setScene(scene);
            window.show();

        }
        public TreeItem<String> makeBranch(String title, TreeItem<String> parent){
            TreeItem<String> item = new TreeItem<>(title);
            item.setExpanded(true);
            parent.getChildren().add(item);
            return item;
        }

    Exception in Application start method
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:567)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:464)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:363)
    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:567)
    at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:1051)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:900)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:195)
    at java.base/java.lang.Thread.run(Thread.java:830)
Caused by: java.lang.IllegalArgumentException: Children: duplicate children added: parent = BorderPane@71e344e8
    at javafx.graphics/javafx.scene.Parent$3.onProposedChange(Parent.java:560)
    at javafx.base/com.sun.javafx.collections.VetoableListDecorator.add(VetoableListDecorator.java:206)
    at javafx.graphics/javafx.scene.layout.BorderPane$BorderPositionProperty.invalidated(BorderPane.java:692)
    at javafx.base/javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:112)
    at javafx.base/javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:147)
    at javafx.graphics/javafx.scene.layout.BorderPane.setCenter(BorderPane.java:268)
    at sample.Main.start(Main.java:70)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:846)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:455)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
Exception running application sample.Main