JavaFX : TreeTableView with CheckComboBoxCell not updating the cell properly

Question

I have the following code to create a TreeTableView with a CheckComboBox in one column.

public class TreeTableViewSample extends Application {

    List<Employee> employees = Arrays.<Employee>asList(
            new Employee("Ethan Williams", ""),
            new Employee("Emma Jones", ""),
            new Employee("Michael Brown", ""),
            new Employee("Anna Black", ""),
            new Employee("Rodger York", ""),
            new Employee("Susan Collins", ""));

    final TreeItem<Employee> root = new TreeItem<>(new Employee("Department", ""));
    final TreeItem<Employee> root1 = new TreeItem<>(new Employee("Executive Department", ""));
    final TreeItem<Employee> root2 = new TreeItem<>(new Employee("Sales Department", ""));

    public static void main(String[] args) {
        Application.launch(TreeTableViewSample.class, args);
    }

    @Override
    public void start(Stage stage) {
        root.setExpanded(true);

        employees.stream().forEach((employee) -> {
            root1.getChildren().add(new TreeItem<>(employee));
            root2.getChildren().add(new TreeItem<>(employee));
        });
        root.getChildren().addAll(root1, root2);
        stage.setTitle("Tree Table View Sample");
        final Scene scene = new Scene(new Group(), 400, 400);
        scene.setFill(Color.LIGHTGRAY);
        Group sceneRoot = (Group) scene.getRoot();

        TreeTableColumn<Employee, String> empColumn
                = new TreeTableColumn<>("Employee");
        empColumn.setPrefWidth(150);
        empColumn.setCellValueFactory(
                (TreeTableColumn.CellDataFeatures<Employee, String> param)
                -> new ReadOnlyStringWrapper(param.getValue().getValue().getName())
        );

        TreeTableColumn<Employee, String> emailColumn
                = new TreeTableColumn<>("Email");
        emailColumn.setPrefWidth(190);
        emailColumn.setEditable(true);

        emailColumn.setCellFactory((TreeTableColumn<Employee, String> p) -> new CheckComboCell());

        emailColumn.setCellValueFactory(
                (TreeTableColumn.CellDataFeatures<Employee, String> param)
                -> new ReadOnlyStringWrapper(param.getValue().getValue().getEmail())
        );

        TreeTableView<Employee> treeTableView = new TreeTableView<>(root);
        treeTableView.setEditable(true);
        treeTableView.getColumns().setAll(empColumn, emailColumn);
        sceneRoot.getChildren().add(treeTableView);
        stage.setScene(scene);
        stage.show();
    }

    public class Employee {

        private SimpleStringProperty name;
        private SimpleStringProperty email;

        public SimpleStringProperty nameProperty() {
            if (name == null) {
                name = new SimpleStringProperty(this, "name");
            }
            return name;
        }

        public SimpleStringProperty emailProperty() {
            if (email == null) {
                email = new SimpleStringProperty(this, "email");
            }
            return email;
        }

        private Employee(String name, String email) {
            this.name = new SimpleStringProperty(name);
            this.email = new SimpleStringProperty(email);
        }

        public String getName() {
            return name.get();
        }

        public void setName(String fName) {
            name.set(fName);
        }

        public String getEmail() {
            return email.get();
        }

        public void setEmail(String fName) {
            email.set(fName);
        }
    }

}

Cell Factory Class

 public class CheckComboCell extends TreeTableCell<TreeTableViewSample.Employee, String> {

    private final ComboBox<CheckComboCellModel> cb = new ComboBox<CheckComboCellModel>() {
        @SuppressWarnings("restriction")
        @Override
        protected javafx.scene.control.Skin<?> createDefaultSkin() {
            return new ComboBoxListViewSkin<CheckComboCellModel>(this) {
                @Override
                protected boolean isHideOnClickEnabled() {
                    return false;
                }
            };
        }
    };

    private final ObservableList<CheckComboCellModel> items = FXCollections.observableArrayList();
    private final Tooltip tooltip = new Tooltip();

    public CheckComboCell() {
        super();
        populateAllocationType();
        cb.setItems(items);

        cb.setCellFactory((ListView<CheckComboCellModel> p) -> new ListCell<CheckComboCellModel>() {
            private final CheckBox checkBox = new CheckBox();
            private BooleanProperty booleanProperty;

            {
                checkBox.setOnAction(e -> getListView().getSelectionModel().select(getItem()));
            }

            @Override
            protected void updateItem(CheckComboCellModel item, boolean empty) {
                super.updateItem(item, empty);
                if (!empty) {
                    checkBox.setText(item.toString());
                    if (booleanProperty != null) {
                        checkBox.selectedProperty().unbindBidirectional(booleanProperty);
                    }
                    booleanProperty = item.checkProperty();
                    checkBox.selectedProperty().bindBidirectional(booleanProperty);
                    setGraphic(checkBox);
                } else {
                    setGraphic(null);
                    setText(null);
                }
            }
        });

        cb.setButtonCell(new ListCell<CheckComboCellModel>() {
            @Override
            protected void updateItem(CheckComboCellModel item, boolean empty) {
                super.updateItem(item, empty);
                String selected = cb.getItems().stream().filter(i -> i.getCheck())
                        .map(i -> i + "").collect(Collectors.joining(","));
                setText(selected);
            }
        });

        setGraphic(null);
    }

    private void populateAllocationType() {
        items.add(new CheckComboCellModel("mail_1"));
        items.add(new CheckComboCellModel("mail_2"));
        items.add(new CheckComboCellModel("mail_3"));
        items.add(new CheckComboCellModel("mail_4"));
        items.add(new CheckComboCellModel("mail_5"));
    }

    @Override
    public void startEdit() {
        if (!isEditable() || !getTreeTableView().isEditable() || !getTableColumn().isEditable()) {
            return;
        }
        super.startEdit();
        setGraphic(cb);
    }

    @Override
    public void commitEdit(String value) {
        super.commitEdit(value);
        setGraphic(null);
        setText(cb.getButtonCell().getText());
    }

    @Override
    public void cancelEdit() {
        super.cancelEdit();
        setGraphic(null);
        setText(cb.getButtonCell().getText());
    }

    @Override
    public void updateItem(String item, boolean empty) {
        super.updateItem(item, empty);
        if (empty) {
            setText(null);
            setGraphic(null);
        } else {
            setText(this.cb.getButtonCell().getText());
            setGraphic(null);
        }
    }
}

Model class

public class CheckComboCellModel {
     private final BooleanProperty check = new SimpleBooleanProperty(false);
    private final StringProperty item = new SimpleStringProperty();

    public CheckComboCellModel() {
    }

    public CheckComboCellModel(String item) {
        this.item.set(item);
    }

    public CheckComboCellModel(String item, Boolean check) {
        this.item.set(item);
        this.check.set(check);
    }

    public BooleanProperty checkProperty() {
        return check;
    }

    public Boolean getCheck() {
        return check.getValue();
    }

    public void setCheck(Boolean value) {
        check.set(value);
    }

    public StringProperty itemProperty() {
        return item;
    }

    public String getItem() {
        return item.getValueSafe();
    }

    public void setItem(String value) {
        item.setValue(value);
    }

    @Override
    public String toString() {
        return item.getValue();
    }

}

With this i get what i expect (i.e) a Column with CheckComboBox. But the issue is:

The application looks like this:

I selected some values

The values are updated in cell

But when the node is minimized

The updated value is displayed in next node. What i am doing wrong ? How can i solve this?