JavaFX ListView:无法向ListCell添加标签

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.ListCell;
import javafx.scene.control.ListView;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class ListViewExample extends Application {

    @Override
    public void start(Stage primaryStage) {
        ListView<String> listView = new ListView<>();
        listView.setCellFactory(param -> new ListCell<String>() {
            @Override
            protected void updateItem(String item, boolean empty) {
                super.updateItem(item, empty);
                if (empty || item == null) {
                    setText(null);
                    setGraphic(null);
                } else {
                    Label label = new Label(item);
                    setGraphic(label);
                }
            }
        });

        listView.getItems().addAll("Item 1", "Item 2", "Item 3");

        VBox root = new VBox(listView);
        Scene scene = new Scene(root, 200, 200);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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