出现异常，帮忙看看！

package src;
import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JToolBar;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
@SuppressWarnings("serial")
public class TestFrame extends JFrame {
JComboBox comboBox;
int index;
   JToolBar toolBar = new JToolBar();//创建一个工具栏
   JButton bNew = new JButton();//创建没有标签和图标的按钮
   JButton bOpen = new JButton();
   JButton bSave = new JButton();
   JButton bbSave=new JButton();
   JButton bComp = new JButton();
   JButton bParse = new JButton();
   JButton bTest = new JButton();
   JButton bSelect = new JButton();
   JTextArea content = new JTextArea();
   JScrollPane jsp = new JScrollPane(content);//创建一个显示指定组件内容的 JScrollPane，只要组件的内容超过视图大小就会自动显示水平和垂直滚动条。
   File curFile = null;
   
     public TestFrame() {
   bNew.setToolTipText("新建");
   bNew.setIcon(new ImageIcon(TestFrame.class.getResource("/com/xyd/java/lesson12/pic/new.gif")));
   bOpen.setToolTipText("打开");
   bOpen.setIcon(new ImageIcon(TestFrame.class.getResource("/com/xyd/java/lesson12/pic/open.gif")));
   bSave.setToolTipText("保存");    
   bSave.setIcon(new ImageIcon(TestFrame.class.getResource("/com/xyd/java/lesson12/pic/save.gif")));
   bbSave.setToolTipText("另存为");
   bbSave.setIcon(new ImageIcon(TestFrame.class.getResource("/com/xyd/java/lesson12/pic/bbsave.gif")));
   bComp.setToolTipText("编译");
   bComp.setIcon(new ImageIcon(TestFrame.class.getResource("/com/xyd/java/lesson12/pic/comp.gif")));
   bParse.setToolTipText("解析");
   bParse.setIcon(new ImageIcon(TestFrame.class.getResource("/com/xyd/java/lesson12/pic/run.gif")));
   bTest.setToolTipText("测试");
   bTest.setIcon(new ImageIcon(TestFrame.class.getResource("/com/xyd/java/lesson12/pic/Test.gif")));
   bSelect.setToolTipText("选择");
   bSelect.setIcon(new ImageIcon(TestFrame.class.getResource("/com/xyd/java/lesson12/pic/select.gif")));
   toolBar.add(bNew);
   toolBar.add(bOpen);
   toolBar.add(bSave);
   toolBar.add(bbSave);
   toolBar.add(bComp);
   toolBar.add(bParse);
   toolBar.add(bTest);
//   toolBar.add(bSelect);
//打开
   bOpen.addActionListener(new ActionListener(){
@Override
public void actionPerformed(ActionEvent e) {
JFileChooser chooser = new JFileChooser("."); //创建一个给定目录的对话框
if(chooser.showOpenDialog(TestFrame.this) == JFileChooser.CANCEL_OPTION) {
return ;//showOpenDialog(Component)显示文件打开对话框，参数为父组件对象
}

curFile = chooser.getSelectedFile();
content.setText("");
try {
BufferedReader br = new BufferedReader(new FileReader(curFile));
String msg = br.readLine();
while(msg != null) {
content.append(msg+"\n");
msg = br.readLine();
}

} catch (IOException e1) {
 JOptionPane.showMessageDialog(TestFrame.this, "打开文件失败！");
 
}
}
   
   });
       
   //解析
       bParse.addActionListener(new ActionListener(){
   public void actionPerformed(ActionEvent e)    {
    StringBuffer str=new StringBuffer("");
     String[]stra=new String[4];
     int i=0;
   try {
Class clazz=Class.forName("src.hello");
Method [] methods=clazz.getDeclaredMethods();
for(Method method: methods){
String modifier=Modifier.toString(method.getModifiers());
Class returnType=method.getReturnType();//返回类型 
Class [] paramTypes=method.getParameterTypes();//参数列表
str=str.append("  "+modifier+"  "+returnType+" "+method.getName()+"(");
 for(int j=0;j<paramTypes.length;j++){
 if(j>0){
 str=str.append(",");
  
       }
  str=str.append(paramTypes[j].getName());  
  
      }
 str=str.append(");");
 String str1=str.toString();
     stra[i]=str1;  i++;
    System.out.println(str1);
 str=new StringBuffer("");
}
// String labels[] = { stra[0], stra[1], stra[2], stra[3]};
// JComboBox comboBox = new JComboBox(labels);   
JComboBox comboBox = new JComboBox();
comboBox.insertItemAt(stra[0], 0);
comboBox.insertItemAt(stra[1], 1);
comboBox.insertItemAt(stra[2], 2);
comboBox.insertItemAt(stra[3], 3);
comboBox.addActionListener(new ActionListener() {   
      public void actionPerformed(ActionEvent e) {  
    index=((JComboBox)e.getSource()).getSelectedIndex() ; 
      System.out.println("Selected index=" + index);   
      }   
    });   
      toolBar.add(comboBox);  
   } catch (ClassNotFoundException e1) {
e1.printStackTrace();
 }
} 
    });
  //测试
       bTest.addActionListener(new ActionListener(){ @Override
public void actionPerformed(ActionEvent e) {
System.out.println(comboBox.getItemAt(index)); 
}
              });
       
   add(toolBar,BorderLayout.NORTH);
   add(jsp);
   setTitle("我的单元测试工具");
   setBounds(400,400,600,600);
   setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   setVisible(true);
   }
      public static void main(String[] args) {
   new TestFrame();
   }
  }
解析完了以后，选择一个方法，按下测试按钮出现这样的异常：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at src.TestFrame$5.actionPerformed(TestFrame.java:248)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:272)
at java.awt.Component.processMouseEvent(Component.java:6038)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
at java.awt.Component.processEvent(Component.java:5803)
at java.awt.Container.processEvent(Container.java:2058)
at java.awt.Component.dispatchEventImpl(Component.java:4410)
at java.awt.Container.dispatchEventImpl(Container.java:2116)
at java.awt.Component.dispatchEvent(Component.java:4240)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
at java.awt.Container.dispatchEventImpl(Container.java:2102)
at java.awt.Window.dispatchEventImpl(Window.java:2429)
at java.awt.Component.dispatchEvent(Component.java:4240)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)是不是System.out.println(comboBox.getItemAt(index)); 这句的问题，帮帮忙看看是哪的问题！谢了！~