Java GUI运行出错，报错信息是java.lang.NullPointerException: Cannot read field "parent" because "comp" is null；

请问以下的代码一运行就会报错，是为什么呢？
报错信息是：java.lang.NullPointerException: Cannot read field "parent" because "comp" is null；
这个怎么解决呢？

import java.awt.BorderLayout;
import java.awt.EventQueue;

import javax.swing.*;
import javax.swing.border.EmptyBorder;

public class SignIn extends JFrame {

private JPanel contentPane;
private JLabel label;
private JLabel label_1;
private JLabel label_2;
private JTextField usernameField;
private JLabel label_3;
private JTextField nameField;
private JLabel label_4;
private JLabel label_5;
private JTextField idNoField;
private JRadioButton idCardButton;
private JRadioButton passportButton;
private JRadioButton twCardButton;
private JPasswordField passwordField1;
private JPasswordField passwordField2;

/**
 * Launch the application.
 */
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                SignIn frame = new SignIn();
                frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}

/**
 * Create the frame.
 */
public SignIn() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBounds(100, 100, 450, 300);
    contentPane = new JPanel();
    contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
    setContentPane(contentPane);
    contentPane.setLayout(null);
    
    label = new JLabel("用户名：");
    label.setBounds(110, 52, 54, 15);
    contentPane.add(label);
    
    usernameField = new JTextField();
    usernameField.setBounds(174, 49, 134, 21);
    contentPane.add(usernameField);
    usernameField.setColumns(10);
    
    label_1 = new JLabel("密码：");
    label_1.setBounds(110, 83, 54, 15);
    contentPane.add(label_1);

    passwordField1 = new JPasswordField();
    passwordField1.setBounds(174, 80, 134, 21);
    contentPane.add(passwordField1);
    
    label_2 = new JLabel("确认密码：");
    label_2.setBounds(110, 113, 60, 15);
    contentPane.add(label_2);

    passwordField2 = new JPasswordField();
    passwordField2.setBounds(174, 110, 134, 21);
    contentPane.add(passwordField2);
    
    label_3 = new JLabel("姓名：");
    label_3.setBounds(112, 143, 52, 15);
    contentPane.add(label_3);
    
    nameField = new JTextField();
    nameField.setBounds(174, 140, 134, 21);
    contentPane.add(nameField);
    nameField.setColumns(20);
    
    label_4 = new JLabel("证件类型：");
    label_4.setBounds(110, 174, 60, 15);
    contentPane.add(label_4);
    
    label_5 = new JLabel("证件号码");
    label_5.setBounds(110, 206, 60, 15);
    contentPane.add(label_5);
    
    idNoField = new JTextField();
    idNoField.setBounds(174, 202, 200, 21);
    contentPane.add(idCardButton);
    idNoField.setColumns(50);
    
    idCardButton = new JRadioButton("身份证");
    idCardButton.setBounds(174, 170, 66, 23);
    contentPane.add(idCardButton);

    passportButton = new JRadioButton("护照");
    passportButton.setBounds(242, 170, 52, 23);
    contentPane.add(passportButton);
    
    twCardButton = new JRadioButton("台胞证");
    twCardButton.setBounds(296, 170, 66, 23);
    contentPane.add(twCardButton);

    ButtonGroup idTypeGroup = new ButtonGroup();
    idTypeGroup.add(idCardButton);
    idTypeGroup.add(passportButton);
    idTypeGroup.add(twCardButton);
    idCardButton.setSelected(true);


}
//获取输入
public String getUsername(){
    String username = usernameField.getText();
    return username;
}
public String getPassword1(){
    String password1 = String.valueOf(passwordField1.getPassword());
    return password1;
}
public String getPassword2(){
    String password2 = String.valueOf(passwordField2.getPassword());
    return password2;
}
public String getTrueName(){
    String name = nameField.getText();
    return name;
}
public String getIdType(){
    boolean idcard = false,passport = false,twcard = false;
    idcard = idCardButton.isSelected();
    passport = passportButton.isSelected();
    twcard = twCardButton.isSelected();
    String idType = null;
    if (idcard){
        idType = "身份证";
    }else if (passport){
        idType = "护照";
    }else if (twcard){
        idType = "台胞证";
    }
    return idType;
}
public String getIdNo(){
    String idNo = idNoField.getText();
    return idNo;
}

}

删除88行的代码，还没有实现不能添加，修改后的代码如下。

import java.awt.EventQueue;
import javax.swing.*;
import javax.swing.border.EmptyBorder;
public class SignIn extends JFrame {
private JPanel contentPane;
private JLabel label;
private JLabel label_1;
private JLabel label_2;
private JTextField usernameField;
private JLabel label_3;
private JTextField nameField;
private JLabel label_4;
private JLabel label_5;
private JTextField idNoField;
private JRadioButton idCardButton;
private JRadioButton passportButton;
private JRadioButton twCardButton;
private JPasswordField passwordField1;
private JPasswordField passwordField2;
 
/**
 * Launch the application.
 */
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                SignIn frame = new SignIn();
                frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}
 
/**
 * Create the frame.
 */
public SignIn() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBounds(100, 100, 450, 300);
    contentPane = new JPanel();
    contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
    setContentPane(contentPane);
    contentPane.setLayout(null);
    
    label = new JLabel("用户名：");
    label.setBounds(110, 52, 54, 15);
    contentPane.add(label);
    
    usernameField = new JTextField();
    usernameField.setBounds(174, 49, 134, 21);
    contentPane.add(usernameField);
    usernameField.setColumns(10);
    
    label_1 = new JLabel("密码：");
    label_1.setBounds(110, 83, 54, 15);
    contentPane.add(label_1);
 
    passwordField1 = new JPasswordField();
    passwordField1.setBounds(174, 80, 134, 21);
    contentPane.add(passwordField1);
    
    label_2 = new JLabel("确认密码：");
    label_2.setBounds(110, 113, 60, 15);
    contentPane.add(label_2);
 
    passwordField2 = new JPasswordField();
    passwordField2.setBounds(174, 110, 134, 21);
    contentPane.add(passwordField2);
    
    label_3 = new JLabel("姓名：");
    label_3.setBounds(112, 143, 52, 15);
    contentPane.add(label_3);
    
    nameField = new JTextField();
    nameField.setBounds(174, 140, 134, 21);
    contentPane.add(nameField);
    nameField.setColumns(20);
    
    label_4 = new JLabel("证件类型：");
    label_4.setBounds(110, 174, 60, 15);
    contentPane.add(label_4);
    
    label_5 = new JLabel("证件号码");
    label_5.setBounds(110, 206, 60, 15);
    contentPane.add(label_5);
    
    idNoField = new JTextField();
    idNoField.setBounds(174, 202, 200, 21);
    idNoField.setColumns(50);
    
    idCardButton = new JRadioButton("身份证");
    idCardButton.setBounds(174, 170, 66, 23);
    contentPane.add(idCardButton);
 
    passportButton = new JRadioButton("护照");
    passportButton.setBounds(242, 170, 52, 23);
    contentPane.add(passportButton);
    
    twCardButton = new JRadioButton("台胞证");
    twCardButton.setBounds(296, 170, 66, 23);
    contentPane.add(twCardButton);
 
    ButtonGroup idTypeGroup = new ButtonGroup();
    idTypeGroup.add(idCardButton);
    idTypeGroup.add(passportButton);
    idTypeGroup.add(twCardButton);
    idCardButton.setSelected(true);
 
}
//获取输入
public String getUsername(){
    String username = usernameField.getText();
    return username;
}
public String getPassword1(){
    String password1 = String.valueOf(passwordField1.getPassword());
    return password1;
}
public String getPassword2(){
    String password2 = String.valueOf(passwordField2.getPassword());
    return password2;
}
public String getTrueName(){
    String name = nameField.getText();
    return name;
}
public String getIdType(){
    boolean idcard = false,passport = false,twcard = false;
    idcard = idCardButton.isSelected();
    passport = passportButton.isSelected();
    twcard = twCardButton.isSelected();
    String idType = null;
    if (idcard){
        idType = "身份证";
    }else if (passport){
        idType = "护照";
    }else if (twcard){
        idType = "台胞证";
    }
    return idType;
}
public String getIdNo(){
    String idNo = idNoField.getText();
    return idNo;
}
}