使用thymeleaf使用Spring Boot将用户添加到Java中的数据库时出现问题

我正在使用本教程：带有tymeleaf的spring boot CRUD应用程序

当我尝试拥有一个用户时，出现了此错误：

2020-03-15 09:51:09.413 ERROR 10168 --- [nio-8080-exec-4] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause

java.lang.NullPointerException：com.CRUD.CRUD_EXAMPLE.controller.UserController.addUser（UserController.java:33）〜[classes /：na]为null

这是代码：'''

 @Controller
public class UserController {
    private UserRepository userRepository;
    @GetMapping("/signup")
    public String showSignUpForm(User user) {
        return "add-user";
    }

    @PostMapping("/adduser")
    public String addUser(@Valid User user, BindingResult result, Model model) {
        if (result.hasErrors()) {
            return "add-user";
        } 


        userRepository.save(user);
        model.addAttribute("users", userRepository.findAll());
        return "index";
    }

    @GetMapping("/edit/{id}")
    public String showUpdateForm(@PathVariable("id") long id, Model model) {
        User user = userRepository.findById(id)
                .orElseThrow(() -> new IllegalArgumentException("Invalid user Id:" + id));

        model.addAttribute("user", user);
        return "update-user";
    }
    @PostMapping("/update/{id}")
    public String updateUser(@PathVariable("id") long id, @Valid User user, 
            BindingResult result, Model model) {
        if (result.hasErrors()) {
            user.setId(id);
            return "update-user";
        }

        userRepository.save(user);
        model.addAttribute("users", userRepository.findAll());
        return "index";
    }

    @GetMapping("/delete/{id}")
    public String deleteUser(@PathVariable("id") long id, Model model) {
        User user = userRepository.findById(id)
                .orElseThrow(() -> new IllegalArgumentException("Invalid user Id:" + id));
        userRepository.delete(user);
        model.addAttribute("users", userRepository.findAll());
        return "index";
    }
}

我的第33行是'userRepository.save（user）';

问题来源：Stack Overflow