Invalid Null Pointer in user-defined Class?

everyone. This is my first time using classes in C++. The instructions in the assignment I'm working on seemed a little ambiguous to me, so I'm including them in case someone more experienced thinks I'm not following them properly in my code.

Anyway, I wrote my program, and it works and creates the output I want based on my perception of the assignment. However, when I run it, an error also pops up that says "Invalid Null Pointer." I've done some research but I can't seem to figure out why this is.

Can someone help me fix my code and help a student understand what they did wrong? Thanks! :)

Assignment

Write a Person class that contains the following fields and methods:

• First Name
• Last Name
• ID Number
• Necessary constructors
• Methods to retu last name, first name, full name, and ID number
• Methods to print last name, first name, and ID number

Write a main program to test your class.

My Program

#include <iostream>
#include <string>

using namespace std;

class Person {
private:
    string FirstName;
    string LastName;
    string FullName;
    int IDNumber;
public:
    void SetFirstName(string);
    void SetLastName(string);
    void SetIDNumber(int);
    string SetFullName(string first, string last);
    string GetFirstName();
    string GetLastName();
    string Method1();
    string Method2();
    int GetIDNumber();
};

void Person::SetFirstName(string first) {
    FirstName = first;
}

void Person::SetLastName(string last) {
    LastName = last;
}

void Person::SetIDNumber(int ID) {
    IDNumber = ID;
}

string Person::SetFullName(string first, string last) {
    FullName = string(first + " " + last);
    retu FullName;
}

string Person::GetFirstName() {
    retu FirstName;
}

string Person::GetLastName() {
    retu LastName;
}


int Person::GetIDNumber() {
    retu IDNumber;
}

string Person::Method1() {
    cout << "LAST NAME: " << LastName << endl;
    cout << "FIRST NAME: " << FirstName << endl;
    cout << "FULL NAME: " << FullName << endl;
    cout << "ID NUMBER: " << IDNumber << endl;
    retu 0;
}

string Person::Method2() {
    cout << "LAST NAME: " << LastName << endl;
    cout << "FIRST NAME: " << FirstName << endl;;
    cout << "ID NUMBER: " << IDNumber << endl;
    retu 0;
}

int main() {
    string firstname, lastname;
    int id;
    char command;
    Person Person;
    cout << "What is the subject's first name? For example: Bob Smith" << endl;
    cin >> firstname >> lastname;
    cout << "What is " << firstname << " " << lastname << "'s ID number?" << endl;
    cin >> id;
    Person.SetFirstName(firstname);
    Person.SetLastName(lastname);
    string fullname = Person.SetFullName(firstname, lastname);
    Person.SetIDNumber(id);
    cout << "COMMANDS:" << endl;
    cout << "f : Retus first name." << endl << "l : Retus last name." << endl << "i : retus ID number." << endl << "n : retus full name." << endl;
    cout << "1 : Retus last name, first name, full name, and ID number." << endl;
    cout << "2 : Retus last name, first name, and ID number." << endl << endl;
    cout << "Please input the letter of your command: ";
    cin >> command;
    switch (command) {
    case 'f':
    case 'F':
        cout << "FIRST NAME: " << Person.GetFirstName() << endl;;
        break;
    case 'l':
    case 'L':
        cout << "LAST NAME: " << Person.GetLastName() << endl;
        break;
    case 'i':
    case 'I':
        cout << "ID NUMBER: " << Person.GetIDNumber() << endl;
        break;
    case 'n':
    case 'N':
        cout << "FULL NAME: " << fullname << endl;
        break;
    case '1':
        Person.Method1();
        cout << endl;
        break;
    case '2':
        Person.Method2();
        cout << endl;
        break;
    default:
        cout << "That is not a valid command." << endl;
    }
}