作为JComboBox元素的JTree不会显示JTree

import java.awt.Component;

import javax.swing.JLabel;
import javax.swing.JList;
import javax.swing.JPanel;
import javax.swing.JTree;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.DefaultTreeModel;
import javax.swing.tree.TreePath;

public class TreeCellRenderer implements ListCellRenderer<Object> {

    private JPanel panel;
    private JLabel label;
    private JTree tree;

    public TreeCellRenderer() {
        panel = new JPanel();
        panel.setLayout(new BorderLayout());
        label = new JLabel();
        tree = new JTree();
        panel.add(label, BorderLayout.NORTH);
        panel.add(tree, BorderLayout.CENTER);
    }

    @Override
    public Component getListCellRendererComponent(JList<?> list, Object value, int index, boolean isSelected,
            boolean cellHasFocus) {
        if (value instanceof DefaultMutableTreeNode) {
            DefaultMutableTreeNode node = (DefaultMutableTreeNode) value;
            label.setText(node.getUserObject().toString());
            tree.setModel(new DefaultTreeModel(node));
        }
        return panel;
    }

}

JComboBox<DefaultMutableTreeNode> comboBox = new JComboBox<>();
comboBox.setRenderer(new TreeCellRenderer());