Java FX：单击按钮后刷新窗格（删除节点后）

Question

假设我有这样的代码：我在其上创建Pane，但createCell和deleteCells按钮。创建另一个窗格，单击createCell按钮时，我会在其子项列表中添加Rectangle。单击deleteCells按钮后，我会从Rectangle的孩子中删除Pane。

public class Main extends Application implements EventHandler<ActionEvent> { 
             //Main.java:17)

    private static int cellId=-1;
    private static Cell[] cells = new Cell[100];

    private BorderPane borderPane;
    private Pane cellsPane;
    private Pane buttonsPane;
    private Pane statsPane;

    Button createCellButton;
    Button deleteCellsButton;

    static int  width= 600;
    static int height=400;

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


    @Override
    public void start(Stage primaryStage) throws Exception {
        primaryStage.setTitle("Hello World");

        borderPane = new BorderPane();
        cellsPane = new Pane();
        buttonsPane = new Pane();
        statsPane = new Pane();

        cellsPane.setStyle("-fx-background-color:  #ffffff; -fx-border-color: black");
        buttonsPane.setStyle("-fx-background-color:  #f2f2f2; -fx-border-color: black");
        statsPane.setStyle("-fx-background-color:  #f2f2f2; -fx-border-color: black");

        createCellButton = new Button();
        deleteCellsButton = new Button();

        createCellButton.setText("Create a cell");
        deleteCellsButton.setText("Delete cells");

        createCellButton.setOnAction(this);
        deleteCellsButton.setOnAction(this);

        buttonsPane.getChildren().add(createCellButton);
        buttonsPane.getChildren().add(deleteCellsButton);

        borderPane.setTop(buttonsPane);

        Rectangle r = new Rectangle(10, 10, 50, 25);
        r.setFill(Color.YELLOW);

        cellsPane.getChildren().add(r);


        borderPane.setCenter(cellsPane);

        statsPane.getChildren().add(new Label("Cells Count: " + cellId +1));
        borderPane.setLeft(statsPane);

        Scene scene = new Scene(borderPane, 600, 400);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    @Override
    public void handle(ActionEvent event)  {
            if (event.getSource()==createCellButton)  {
                    cells[++cellId] = createCell();
                    cellsPane.getChildren().add(cells[cellId].getModel());
                    System.out.println("Cell created: " + cells[cellId].getAge() + ", " + cells[cellId].getColor());
            }
            else if (event.getSource() == deleteCellsButton) {
                    deleteCells(); //Main.java:88
                    System.out.println("Cells deleted");
            }
            else  {
                    System.out.println("Unknown button");
            }
    }

    private Cell createCell()  {
            return new Cell(cellId, width, height);
    }

        private void deleteCells()  {
        for (Cell cell: cells)  {
                System.out.println("removing " + cell.getAge()); 
                                                          //Main.java:102
                cellsPane.getChildren().remove(cell.getModel());
                System.out.println("cell " + cell.getAge() + " removed");
        }
}

Cell课程：

    class Cell  {
        private int age;
        private Color color;
        private Rectangle model;

        public Cell(int id, int width, int height)  {
                age = id;

                Random rand = new Random();

                int x = rand.nextInt(width+1);
                int y = rand.nextInt(height+1);

                color = Color.YELLOW;
                model = new Rectangle(x, y, 50, 25);
                model.setFill(color);
        }

        public Rectangle getModel()  {
                return model;
        }

        public int getAge()  {
            return age;
        }

        public Color getColor()  {
            return color;
        }
}

但是，点击deleteCells后，窗口中仍然可以看到之前的矩形。我如何解决它？如何在单击按钮后刷新Pane？

修改

java.lang.NullPointerException
   at sample.Main.deleteCells(Main.java:102)
   at sample.Main.handle(Main.java:88)
   at sample.Main.handle(Main.java:17)

System.out.println输出：

Cell created: 0, 0xffff00ff
Cell created: 1, 0xffff00ff
Cell created: 2, 0xffff00ff
Cell created: 3, 0xffff00ff
Cell created: 4, 0xffff00ff
Cell created: 5, 0xffff00ff
Cell created: 6, 0xffff00ff
Cell created: 7, 0xffff00ff
Cell created: 8, 0xffff00ff
Cell created: 9, 0xffff00ff
Cell created: 10, 0xffff00ff
removing 0
cell 0 removed
removing 1
cell 1 removed
removing 2
cell 2 removed
removing 3
cell 3 removed
removing 4
cell 4 removed
removing 5
cell 5 removed
removing 6
cell 6 removed
removing 7
cell 7 removed
removing 8
cell 8 removed
removing 9
cell 9 removed
removing 10
cell 10 removed