Passing JSON data using POST in Spring Web MVC

Question

I'm trying to create a simple REST service using Spring Web MVC and the JSON library https://mvnrepository.com/artifact/org.json/json/20210307. However, I've problems when passing data in JSON using POST, since Java raises an NullPointerException when the constructor of the data is invoked.

The REST controller that I created is this one:

        @RestController @RequestMapping(path = "users/")
        public class UserController {

            @PostMapping(path = "/", consumes = MediaType.APPLICATION_JSON_VALUE, produces = MediaType.APPLICATION_JSON_VALUE)
            public String addUser(@RequestBody User user) {
                return user.toString();
            }

        }

User class is defined in this way:

public class User extends Item implements Jsonable{
    
    private String firstName;
    private String lastName;
    private LocalDate dateOfBirth;
    private String taxCode;
    
    private static int INCREMENTAL_ID = 0;
    
    public User(String firstName, String lastName) {
        super(INCREMENTAL_ID++);
        this.firstName = firstName;
        this.lastName = lastName;
        this.dateOfBirth = LocalDate.now();
        this.taxCode = this.generateTaxCode();
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFistName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public LocalDate getDateOfBirth() {
        return dateOfBirth;
    }

    public void setDateOfBirth(String dateOfBirth) {
        this.dateOfBirth = LocalDate.parse(dateOfBirth);
    }

    public String getTaxCode() {
        return taxCode;
    }

    public void setTaxCode(String taxCode) {
        this.taxCode = taxCode;
    }
    
    private String generateTaxCode() {
        StringBuilder strBuilder = new StringBuilder();
        strBuilder.append(this.getFirstName().toUpperCase().substring(0, 3))
            .append(this.getLastName().toUpperCase().substring(0, 3))
            .append(this.getDateOfBirth().toString())
            .append((int)(Math.random()*100));
        return strBuilder.toString();
    }

    @Override
    public String toString() {
        return super.toString() + " " + firstName + " " + lastName + " " + dateOfBirth
                + " " + taxCode;
    }

    @Override
    public JSONObject toJson() {
        return super.toJson().put("firstName", this.getFirstName())
            .put("lastName", this.getLastName())
            .put("dateOfBirth", this.getDateOfBirth().toString())
            .put("taxCode", this.getTaxCode());
    }
}

Then, passing the data in this way

{
     "firstName": "Example",
     "lastName": "Example"
}

The server raises the following error:

Constructor threw exception; nested exception is java.lang.NullPointerException] with root cause
java.lang.NullPointerException
    at ...User.generateTaxCode(User.java:61)
    at ...User.<init>(User.java:24)

How can I pass data in a valid JSON format to the controller, is there any other Java Annotation in addUser method that makes this correctly?