Fatal error: Uncaught Error: Call to a member function get () on null

I have been with this problem for a little more than a day, I checked the possible similarity of my question with others, but the truth, none of them worked for me.

I'm having the following error in PHP: Fatal error: Uncaught Error: Call to a member function get() on null

File Url.php , line 28.

Url.php :

<?php

class Url extends App {

 private $db;
 private static $alias = null;
 private static $mcpe_list = null;
 private static $url2 = null;
 private static $clicks = 0;
 private static $name = null;
 private static $password = null;

 public function __construct() {
  $this->db = json_decode(file_get_contents('/../' . $this->config->get('DB_PATH')), true);
 }

}

App.php :

<?php

class App {

 protected $error;
 protected $url;
 protected $config;

 const CFG_PATH = 'private/config.json';
 const VAL_ERRORS = [400, 401, 403, 404, 500];

 public function __construct() {
  $this->error = new Error();
  $this->url = new Url();
  $this->config = new Config(self::CFG_PATH);
 }

}

The error, $this->config->get('DB_PATH') occurs in the constructor of class Url.

Instances are created in the App class and as Url extends App, the protected properties are inherited.

Thanks for your answers.