改错 java小代码

Question

请帮忙看看下面代码错在哪里，请指出出错的原因，谢谢！
编译可以通过，但运行时报NullPointerException 位置是25行，也就是代码中红色字体那行。
代码如下
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class N extends JFrame implements ItemListener,ActionListener
{
String score[][];
String colNames[]={"学号","姓名","课程名","成绩"};
String strCname,strTname;
JButton Bconfirm,Bsubmit,BsetNull,Bexit,Bmodify;
JComboBox CBcname,CBteacher;
Container con=getContentPane();
JLabel Lcname,Lteacher;
JPanel topPanel,downPanel; public void Init()
{
this.setSize(600,400);
Lcname=new JLabel("请选择课程:");
Lteacher=new JLabel("请选择教师名");
CBcname=new JComboBox();
CBteacher=new JComboBox();
Bconfirm=new JButton("确定");
Bexit=new JButton("退出");
GridBagLayout gridbag=new GridBagLayout();
GridBagConstraints gridbagcon;
topPanel.setLayout(gridbag);
gridbagcon=new GridBagConstraints();
gridbagcon.gridx=0;gridbagcon.gridy=0;
        gridbagcon.insets=new Insets(10,1,10,30);
gridbag.setConstraints(Lcname,gridbagcon);
topPanel.add(Lcname); gridbagcon.gridx=1;gridbagcon.gridy=0;
        gridbagcon.insets=new Insets(10,1,10,30);
gridbag.setConstraints(CBcname,gridbagcon);
topPanel.add(CBcname);
CBcname.addItemListener(this); gridbagcon.gridx=2;gridbagcon.gridy=0;
        gridbagcon.insets=new Insets(10,30,10,1);
gridbag.setConstraints(Lteacher,gridbagcon);
topPanel.add(Lteacher); gridbagcon.gridx=3;gridbagcon.gridy=0;
        gridbagcon.insets=new Insets(10,1,10,30);
gridbag.setConstraints(CBteacher,gridbagcon);
topPanel.add(CBteacher);
CBteacher.addItemListener(this);
con.add(topPanel,BorderLayout.CENTER);
        
downPanel.setLayout(new FlowLayout());
        downPanel.add(Bconfirm);
Bconfirm.addActionListener(this);
downPanel.add(Bexit);
Bexit.addActionListener(this);
con.add(downPanel,BorderLayout.SOUTH);
} public void itemStateChanged(ItemEvent ie)
{
} public void actionPerformed(ActionEvent ae)
{
} public static void main(String args[])
{
N mgra=new N();
mgra.Init();
mgra.pack();
mgra.setVisible(true);
}
}

Answer 1

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;public class N extends JFrame implements ItemListener, ActionListener {
String score[][]; String colNames[] = { "学号", "姓名", "课程名", "成绩" }; String strCname, strTname; JButton Bconfirm, Bsubmit, BsetNull, Bexit, Bmodify; JComboBox CBcname, CBteacher; Container con = getContentPane(); JLabel Lcname, Lteacher; JPanel topPanel, downPanel; public void Init() {
this.setSize(600, 400);
Lcname = new JLabel("请选择课程:");
Lteacher = new JLabel("请选择教师名");
CBcname = new JComboBox();
CBteacher = new JComboBox();
Bconfirm = new JButton("确定");
Bexit = new JButton("退出");
GridBagLayout gridbag = new GridBagLayout();
GridBagConstraints gridbagcon;
//只是定义了topPanel为JPanel类型，但并没有构造实际对象给它，所以topPanel指向null
//new 个JPanel对象
topPanel = new JPanel();
topPanel.setLayout(gridbag);
gridbagcon = new GridBagConstraints();
gridbagcon.gridx = 0;
gridbagcon.gridy = 0;
gridbagcon.insets = new Insets(10, 1, 10, 30);
gridbag.setConstraints(Lcname, gridbagcon);
topPanel.add(Lcname); gridbagcon.gridx = 1;
gridbagcon.gridy = 0;
gridbagcon.insets = new Insets(10, 1, 10, 30);
gridbag.setConstraints(CBcname, gridbagcon);
topPanel.add(CBcname);
CBcname.addItemListener(this); gridbagcon.gridx = 2;
gridbagcon.gridy = 0;
gridbagcon.insets = new Insets(10, 30, 10, 1);
gridbag.setConstraints(Lteacher, gridbagcon);
topPanel.add(Lteacher); gridbagcon.gridx = 3;
gridbagcon.gridy = 0;
gridbagcon.insets = new Insets(10, 1, 10, 30);
gridbag.setConstraints(CBteacher, gridbagcon);
topPanel.add(CBteacher);
CBteacher.addItemListener(this);
con.add(topPanel, BorderLayout.CENTER);
//只是定义了downPanel为JPanel类型，但并没有构造实际对象给它，所以downPanel指向null
//new 个JPanel对象
downPanel = new JPanel();
downPanel.setLayout(new FlowLayout());
downPanel.add(Bconfirm);
Bconfirm.addActionListener(this);
downPanel.add(Bexit);
Bexit.addActionListener(this);
con.add(downPanel, BorderLayout.SOUTH);
} public void itemStateChanged(ItemEvent ie) {
} public void actionPerformed(ActionEvent ae) {
} public static void main(String args[]) {
N mgra = new N();
mgra.Init();
mgra.pack();
mgra.setVisible(true);
}
}
哥们，你只是定义了变量，但并没有构造对象。