ChoiceBox与图像/文本和分隔符

Question

这个问题解决了问题：带有图像和文字的ChocieBox： JavaFX: ChoiceBox with image and text 但是当我在ChoiceBox中添加一个Separator时，我得到了一个Exception。 这是代码：

statusComboBox.setItems(FXCollections.observableArrayList("open",new Separator(), "save", "exit"));

    statusComboBox.setCellFactory(new Callback<ListView<String>, ListCell<String>>() {
        public ListCell<String> call(ListView<String> p) {
            return new ListCell<String>() {
                @Override
                protected void updateItem(String item, boolean empty) {
                    super.updateItem(item, empty);
                    setText(item);
                    if (item == null || empty) {
                        setGraphic(null);
                    } else {
                        Image icon;
                        try {
                            int iconNumber = this.getIndex() + 1;
                            String iconPath = "/icon_" + iconNumber + ".png";
                            icon = new Image(getClass().getResourceAsStream(iconPath));
                        } catch (NullPointerException ex) {
                            // in case the above image doesn't exist, use a default one
                            String iconPath = "/icon_na.png";
                            icon = new Image(getClass().getResourceAsStream(iconPath));
                        }
                        ImageView iconImageView = new ImageView(icon);
                        iconImageView.setFitHeight(30);
                        iconImageView.setPreserveRatio(true);
                        setGraphic(iconImageView);
                    }
                }
            };
        }
    });

，这是例外：

Exception in thread "JavaFX Application Thread" java.lang.ClassCastException: javafx.scene.control.Separator cannot be cast to java.lang.String
at dumaisoft.SampleController$2$1.updateItem(SampleController.java:49)
at javafx.scene.control.ListCell.updateItem(ListCell.java:471)
at javafx.scene.control.ListCell.indexChanged(ListCell.java:330)
at javafx.scene.control.IndexedCell.updateIndex(IndexedCell.java:116)
at com.sun.javafx.scene.control.skin.VirtualFlow.setCellIndex(VirtualFlow.java:1957)
at com.sun.javafx.scene.control.skin.VirtualFlow.getCell(VirtualFlow.java:1797)
at com.sun.javafx.scene.control.skin.VirtualFlow.getCellLength(VirtualFlow.java:1879)
at com.sun.javafx.scene.control.skin.VirtualContainerBase.getVirtualFlowPreferredHeight(VirtualContainerBase.java:101)
at com.sun.javafx.scene.control.skin.ComboBoxListViewSkin.getListViewPrefHeight(ComboBoxListViewSkin.java:530)
at com.sun.javafx.scene.control.skin.ComboBoxListViewSkin.access$300(ComboBoxListViewSkin.java:57)
at com.sun.javafx.scene.control.skin.ComboBoxListViewSkin$3.computePrefHeight(ComboBoxListViewSkin.java:472)
at javafx.scene.Parent.prefHeight(Parent.java:935)
at javafx.scene.layout.Region.prefHeight(Region.java:1435)

我该如何解决？