SpringBootで掲示板機能を実現

package com.codessquad.qna.domain;

public class User {

    private String userId;
    private String password;
    private String name;
    private String email;


    @Override
    public String toString() {
    //투스트링 메서드 생략
    }
    
    //게터세터 생략

}

しかし、不思議なことに、ドメインにコードを追加するとコントローラが見つかりません。

package com.codessquad.qna.domain;

public class User {

    private String userId;
    private String password;
    private String name;
    private String email;

    public User(User copyUser) {
        this.userId = copyUser.userId;
        this.password = copyUser.password;
        this.name = copyUser.name;
        this.email = copyUser.email;
    }

    @Override
    public String toString() {
    //투스트링 메서드 생략
    }
    
    //게터세터 생략

}

コンソールはこうです。

 @PostMapping("/usercreate")
    public String create(User user) {
        userList.add(user);

        return "redirect:/users/list";
    }

ここで質問があります。domainにコンストラクション関数が追加された理由を知りたいのですが、java springは次のエラーメッセージを送信します。

Whitelabel Error Page
This application has no explicit mapping for /error, so you are seeing this as a fallback.

Tue Mar 09 12:39:13 KST 2021
There was an unexpected error (type=Internal Server Error, status=500).
Failed to instantiate [com.codessquad.qna.domain.User]: Constructor threw exception; nested exception is java.lang.NullPointerException
org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.codessquad.qna.domain.User]: Constructor threw exception; nested exception is java.lang.NullPointerException