问为什么javafx8 UI线程在变更列表中表现得很奇怪？

EN

numberOfBars.getSelectionModel().selectedItemProperty().addListener((observable, oldValue, newValue) -> {
   createData(newValue);
   // It works fine if I add Thread.sleep(1000) here. Why?
   createBarChart();
});

public class Main extends Application {
    private XYChart.Series<String, Number> data = new XYChart.Series<>();
    private VBox vBox = new VBox();
    private Pane bcContainer = new Pane();
    @Override public void start(Stage stage){
        ChoiceBox<Integer> numberOfBars = new ChoiceBox<>();
        ObservableList<Integer> items = FXCollections.observableArrayList(5, 10);
        numberOfBars.setItems(items);
        vBox.getChildren().addAll(new Label("Number of bars: "), numberOfBars, new Separator(), bcContainer);
        numberOfBars.getSelectionModel().selectedItemProperty().addListener((observable, oldValue, newValue) -> {
            createData(newValue);
            // It works fine if I add Thread.sleep(1000) here. Why?
            createBarChart();
        });
        Scene scene  = new Scene(vBox,800,600);
        stage.setScene(scene);
        stage.show();
    }
    private void createData(int bars){
        ObservableList<XYChart.Data<String, Number>> observableList = FXCollections.observableArrayList();
        for (int i=0; i<bars; i++) {
            observableList.add(new XYChart.Data<>("bar" + Integer.toString(i), i+1));
        }
        data.setData(observableList);
    }
    private void createBarChart(){
        BarChart<String, Number> barChart= new BarChart<>(new CategoryAxis(), new NumberAxis());
        barChart.setLegendVisible(false);
        barChart.setPrefSize(800, 500);
        barChart.getData().add(data);
        bcContainer.getChildren().clear();
        bcContainer.getChildren().add(barChart);
    }
    public static void main(String[] args) {
        launch(args);
    }
}