Problem adding user to database in java using spring boot with thymeleaf

I was using this tutorial : spring boot CRUD application with tymeleaf

The problem happen when I try to had a user, I got this error :

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: null
at com.CRUD.CRUD_EXAMPLE.controller.UserController.addUser(UserController.java:33) ~[classes/:na]

Here's the code :
'''

    @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";
}
}

'''

My line 33 is 'userRepository.save(user)';

As it says you, The object User user that comes to the method is null, or one of its field is null.
Make sure that this objects contains all data coming from view.
Check also if all the field in your html file are well mapped to the field of the User object.

Please mark your UserRepository implementation class with the @Repository annotation, like this:

@Repository
public class UserRepositoryImpl implements UserRepository {

Then inject the dependency via the @Autowired annotation in your controller like this:

@Controller
public class UserController {
private UserRepository userRepository;
@Autowired
public UserController(UserRepository userRepository) {
this.userRepository = userRepository;
}

This will fix your NullPointerException, because Spring will now create and inject this dependency for you.