Exception in Thread "main " java.lang.NullPointerException 运行出错哪位大神帮忙解决下

package UserCardLayoutFrame;import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.net.URL;import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.WindowConstants;public class UserCardLayoutFrame extends JFrame {
 
private JTextField txtId;
private URL url = null;
private Icon icon = null;
private JPanel imgPanel;
private CardLayout cardLayout;

public UserCardLayoutFrame(){
setTitle("猜一个成语");
setBounds(100,100,365,252);
setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
final JPanel northPanel = new JPanel();
getContentPane().add(northPanel,BorderLayout.NORTH);
final JLabel label = new JLabel();
label.setText("输入图片编号：");
northPanel.add(label);
txtId = new JTextField(10);
northPanel.add(txtId);
imgPanel.setLayout(cardLayout);
getContentPane().add(imgPanel,BorderLayout.CENTER);
for(int i=1;i<=6;i++){
url = UserCardLayoutFrame.class.getResource("/image/"+i+".gif");
icon = new ImageIcon(url);
imgPanel.add(String.valueOf(i), new JLabel(icon));

}
final JButton button = new JButton();
button.addActionListener(new ActionListener(){
public void actionPerformed(final ActionEvent arg0){
String id = txtId.getText();
cardLayout.show(imgPanel, id);

}
});
button.setText("显示照片");
northPanel.add(button);
final JPanel southPanel = new JPanel();
getContentPane().add(southPanel,BorderLayout.SOUTH);
final JButton dButton = new JButton();
dButton.addActionListener(new ActionListener(){
public void actionPerformed(final ActionEvent arg0){
cardLayout.first(imgPanel);

}
});
dButton.setText("第一张");
southPanel.add(dButton);
final JButton fButton = new JButton();
fButton.addActionListener(new ActionListener(){
public void actionPerformed(final ActionEvent arg0){
cardLayout.previous(imgPanel);
}
});
fButton.setText("上一张");
southPanel.add(fButton);

final JButton gButton = new JButton();
gButton.addActionListener(new ActionListener(){
public void actionPerformed(final ActionEvent arg0){
cardLayout.next(imgPanel);
}
});
gButton.setText("下一张");
southPanel.add(gButton);
final JButton button_1 = new JButton();
button_1.addActionListener(new ActionListener(){
public void actionPerformed(final ActionEvent arg0){
cardLayout.last(imgPanel);
}
});
button_1.setText("最后一张");
southPanel.add(button_1);
//southPanel.add(button_1);

}
 public static void main(String[] args) {
 UserCardLayoutFrame frame = new UserCardLayoutFrame();
 frame.setVisible(true);
 
 }
}报错是这样得：
Exception in thread "main" java.lang.NullPointerException
at UserCardLayoutFrame.UserCardLayoutFrame.<init>(UserCardLayoutFrame.java:37)
at UserCardLayoutFrame.UserCardLayoutFrame.main(UserCardLayoutFrame.java:95)
请问怎么解决下