急用！！！！
有这么一个设置为什么出现下面的错误？
Exception   in   thread   "main "   java.lang.IllegalArgumentException:   adding   a   window   t
o   a   container
                at   java.awt.Container.addImpl(Container.java:1010)
                at   java.awt.Container.add(Container.java:364)
                at   JieMian. <init> (JieMian.java:46)
                at   JieMian.main(JieMian.java:61)
我是在一个面板里面添加了一个窗口
import   java.awt.*;
import   java.awt.event.*;
import   javax.swing.*;
import   java.io.*;
public   class   JieMian   extends   JFrame   implements   ActionListener
{

ShuJuJieMianGUI   shuju=null;
//SanWeiTuXingGUI   tuxing=null;
JMenuBar   mb;
JMenu   menu;
JMenuItem   Reading,Drawing,exit;
JLabel   label=null;
JPanel   pCenter;
CardLayout   layout=null;
public   JieMian()
{
mb=new   JMenuBar();
mb.setOpaque(true);
menu=new   JMenu( "功能选择 ");//gong   neng   xuan   ze  
Reading=new   JMenuItem( "读入DEM数据 ");//du   ru   DEM   shu   ju
Drawing=new   JMenuItem( "DEM数据三位显示 ");//DEM   shu   ju   de   san   wei   xian   shi
exit=new   JMenuItem( "退出 ");//tui   chu   an   liu

menu.add(Reading);
menu.add(Drawing);
menu.addSeparator();
menu.add(exit);
mb.add(menu);
setJMenuBar(mb);

label=new   JLabel( "我的三维DEM显示程序 ",JLabel.CENTER);
label.setFont(new   Font( "楷体_GB2312 ",Font.BOLD,30));//she   ji   zi   ti
label.setForeground(Color.blue);

shuju=new   ShuJuJieMianGUI();

Container   c=this.getContentPane();//rong   qi
pCenter=new   JPanel();//mian   ban
layout=new   CardLayout();//bu   ju   ge   shi
c.add(pCenter, "Center ");//rong   qi   li   mian   tian   jia   mian   ban
pCenter.setLayout(layout);//mian   ban   de   bu   ju   ge   shi
pCenter.add( "WELCOME   MODE ",label);
pCenter.add( "READING   MODE ",shuju);//错误出在这个地方
//pCenter.add( "DRAWING   MODE ",tuxing);

this.setBounds(0,0,800,650);
this.setTitle( "我的毕业论文程序 ");//she   zhi   biao   ti  
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setVisible(true);
this.validate();

exit.addActionListener(this);
Reading.addActionListener(this);

}  
public   static   void   main(String[]   args)
{
new   JieMian();  
}  
public   void   actionPerformed(ActionEvent   e)
{
if(e.getSource()==exit)
{
System.exit(0);
}
else   if(e.getSource()==Reading)
{
layout.show(pCenter, "READING   MODE ");
}
}

}
class   ShuJuJieMianGUI   extends   JFrame
{
Object[][]   ShuJuInfo={{0,0,0},{0,0,0},{0,0,0},{0,0,0}};
        Object[]   ShuJuName={ "X ", "Y ", "Z "};
JTable   table;
JScrollPane   sp;