Error when using __construct in a child of a MySQL class (it is solved by not using __construct but I need it if or if)

I have two classes: The first is called "MySQL" and contains all the functions for the queries (in addition to the ones that the API "MySQLi" brings.) Use __construct to create the MySQLi object. The second (Users), which is the daughter of the previous one, uses a __construct to instantiate other necessary classes.

When using __construct in the "Users" class, it returns an error: Fatal error: Uncaught Error: Call to a member function query () on null in D: \ Lautaro \ Applications \ Programs \ Xampp \ htdocs \ src \ classes \ MySQL.php.

If I delete __construct and its sentences, the error is solved, but I need YES or SI to make those instances.

MySQL.php

namespace Brax\BBDD;

  require_once $_SERVER['DOCUMENT_ROOT'].'/config/cfg.php';

  class MySQL {
    private $server;
    private $username;
    private $password;
    private $database;
    public $mysqli; // Ver si la visibilidad afecta a los traits

    public function __construct() {
      $this->server   = 'localhost';
      $this->username = 'root';
      $this->password = '';
      $this->database = 'db';

      $this->mysqli = new \mysqli($this->server, $this->username, $this->password, $this->database);
      $this->mysqli->set_charset('utf8');
    }

    /**
     * This function will make a query to the DB.
     * @param  string $sql       [It's a SQL string]
     * @return object|false      [If succesfull, it will return an object, else, it will return false]
     */
    protected function query(string $sql) {
      if (!empty($sql)) {
        if ($response = $this->mysqli->query($sql)) {
          if (DEBUG === true) {
            echo '['.HOUR.'] MySQLi query() DEBUG [SQL Executed]: '.$sql.PHP_EOL;
          }

          return $response;
        } else {
          if (DEBUG === true) {
            echo '['.HOUR.'] MySQLi query() DEBUG [SQL Executed with ERROR * ]: '.$sql.PHP_EOL;
          }

          return false;
        }
      } else {
        if (DEBUG === true) {
          echo '['.HOUR.'] MySQLi Query DEBUG [SQL]: The query is empty.'.PHP_EOL;
        }

        return false;
      }
    }

Users.php

namespace Brax\Usuarios;
  require_once $_SERVER['DOCUMENT_ROOT'].'/config/cfg.php';
  spl_autoload_register('Brax\autoload');

  class Usuarios extends \Brax\BBDD\MySQL {
    use \Brax\Traits\CRUDTrait;

    private $Panel,
        $Accesos,
        $Referidos,
        $GameServers;

    public function __construct() {
      $this->Panel       = new \Brax\Panel\Panel();
      $this->Accesos     = new Accesos\Accesos();
      $this->Referidos   = new \Brax\Referidos\PostsReferidos();
      $this->GameServers = new \Brax\GameServers\GameServers();
    }
}

The Users class should work with __construct, and not return the error explained above.