如何自定义 JComboBox 以便弹出窗口是 JTree（而不是列表）？

import javax.swing.*;
import javax.swing.plaf.basic.ComboPopup;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.DefaultTreeModel;
import javax.swing.tree.TreeModel;
import java.awt.*;

public class CustomJComboBox extends JComboBox {

    private JTree tree;

    public CustomJComboBox() {
        setEditable(true);
        setModel(new DefaultComboBoxModel());
        setRenderer(new CustomListCellRenderer());
        setUI(new CustomComboBoxUI());

        DefaultMutableTreeNode rootNode = new DefaultMutableTreeNode("Root");
        DefaultMutableTreeNode childNode1 = new DefaultMutableTreeNode("Child 1");
        DefaultMutableTreeNode childNode2 = new DefaultMutableTreeNode("Child 2");
        DefaultMutableTreeNode childNode3 = new DefaultMutableTreeNode("Child 3");
        rootNode.add(childNode1);
        rootNode.add(childNode2);
        rootNode.add(childNode3);

        TreeModel model = new DefaultTreeModel(rootNode);
        tree = new JTree(model);
    }

    private class CustomListCellRenderer extends DefaultListCellRenderer {
        @Override
        public Component getListCellRendererComponent(JList list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
            JLabel label = (JLabel) super.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
            label.setText("Custom JComboBox");
            return label;
        }
    }

    private class CustomComboBoxUI extends BasicComboBoxUI {
        @Override
        protected ComboPopup createPopup() {
            return new CustomComboPopup(comboBox);
        }
    }

    private class CustomComboPopup extends BasicComboPopup {
        public CustomComboPopup(JComboBox combo) {
            super(combo);
        }

        @Override
        public void show() {
            if (isVisible()) {
                hide();
            } else {
                Insets insets = comboBox.getInsets();
                Dimension popupSize = comboBox.getSize();
                int x = insets.left;
                int y = insets.top + comboBox.getHeight();
                setLocation(x, y);
                setSize(popupSize.width, 200);
                tree.setRootVisible(true);
                tree.setShowsRootHandles(true);
                add(new JScrollPane(tree));
                setVisible(true);
                setFocusable(true);
                comboBox.firePopupMenuWillBecomeVisible();
            }
        }
    }
}

import javax.swing.*;

public class Main {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            JFrame frame = new JFrame("Custom JComboBox with JTree");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setSize(400, 300);

            CustomJComboBox comboBox = new CustomJComboBox();
            frame.add(comboBox);

            frame.setVisible(true);
        });
    }
}