java显示问题求教

一个程序
添加了个JTree
起初不显示
要最小化一下以后才显示
求高人指点！！

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package xitong_main_bumen;

import javax.swing.*;
import javax.swing.event.TreeSelectionEvent;
import javax.swing.event.TreeSelectionListener;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.TreePath;
import richang.table_Model_zichan;
import java.awt.event.*;
import java.sql.*;

public class zichan_bumen extends JFrame implements ActionListener,
TreeSelectionListener {
private JPanel panel;
private JFrame frame;
private JScrollPane sp;
private JScrollPane spt;
private table_Model_zichan model;
private JTable table;
private JButton button_add;
private JButton button_del;
private JButton button_update;
private int id;
public JTree tree;

public zichan_bumen(JFrame frame, JPanel panel, int id) {
this.id = id;
this.panel = panel;
panel.setSize(790, 450);
panel.setLayout(null);
panel.removeAll();
this.frame = frame;
model = new table_Model_zichan(20);
table = new JTable(model);
table.setAutoResizeMode(JTable.AUTO_RESIZE_OFF);
table.getColumnModel().getColumn(0).setPreferredWidth(40);
table.getColumnModel().getColumn(1).setPreferredWidth(100);
table.getColumnMode