How to get details of logged in user with Identity

I understand that the Identity is returned from my class MyAuthenticator which implements NS\IAuthenticator.

I can get the user id of the logged in user with $this->getUser()->getId() however,
I cannot get the username of the logged in user with $this->getUser()->getIdentity()->username.

Is there something in the authenticator which needs to be loaded to populate the identity?

My Authenticator is shown here:

<?php

namespace App\Model\SiteAuthenticator;

use \Nette\Security\IIdentity;
use \Nette\Security\Passwords;
use \Nette\Security\IAuthenticator;

class SiteAuthenticator implements IAuthenticator
{
        public function __construct(\Nette\Database\Context $database, \Nette\Security\Passwords $passwords)
        {
                $this->database = $database;
                $this->passwords = $passwords;
        }

        public function authenticate(array $credentials): \Nette\Security\IIdentity
        {
                [$username, $password] = $credentials;

                $row = $this->database->table('users')
                        ->where('username', $username)->fetch();

                if (!$row) {
                        throw new \Nette\Security\AuthenticationException('User not found.');
                }

                if (!$this->passwords->verify($password, $row->password)) {
                        throw new \Nette\Security\AuthenticationException('Invalid password.');
                }

                return new \Nette\Security\Identity($row->id, ['username' => $row->username]);
        }
}
?>