Nullsafe operator

I am learning about nullsafe operator. I have written the below code:

<?php

class User 
{
    public $isoCode = '259874';

    public function getAddress() : string {
        return 'Dhaka';
    }

    public function getCountry() : string {
        return 'Bangladesh';
    }
}

$user = new User();
echo $user->getAddress()?->getCountry()?->isoCode;

But why this getAddress() giving me this error?

Fatal error: Uncaught Error: Call to a member function getCountry() on string in C:\xampp\htdocs\php8\nullsafe.php:18 Stack trace: #0 {main} thrown in C:\xampp\htdocs\php8\nullsafe.php on line 18