Erro Trying to access array offset on value of type null

Olá a todos, tenho uma aplicação pequena, que roda normalmente no php5 porém ao mudar a versão do php para 7 a mesma está apresentando a seguinte mensagem de erro

Notice: Trying to access array offset on value of type null in

<?php
 /*
 * APP CORE CLASS
 * Creates URL & Loads Core Controller
 * URL Format - /controller/method/param1/param2
 */
 class Core {
 // Set Defaults
 protected $currentController = 'Pages'; // Default controller
 protected $currentMethod = 'index'; // Default method
 protected $params = []; // Set initial empty params array

 public function __construct(){
 $url = $this->getUrl();
 // Look in controllers folder for controller
 if(file_exists('../app/controllers/'.ucwords($url[0]).'.php')){
 // If exists, set as controller
 $this->currentController = ucwords($url[0]);
 // Unset 0 index
 unset($url[0]);
 }

 // Require the current controller
 require_once('../app/controllers/' . $this->currentController . '.php');

 // Instantiate the current controller
 $this->currentController = new $this->currentController;

 // Check if second part of url is set (method)
 if(isset($url[1])){
 // Check if method/function exists in current controller class
 if(method_exists($this->currentController, $url[1])){
 // Set current method if it exsists
 $this->currentMethod = $url[1];
 // Unset 1 index
 unset($url[1]);
 }
 }

 // Get params - Any values left over in url are params
 $this->params = $url ? array_values($url) : [];

 // Call a callback with an array of parameters
 call_user_func_array([$this->currentController, $this->currentMethod], $this->params);
 }

 // Construct URL From $_GET['url']
 public function getUrl(){
 if(isset($_GET['url'])){
 $url = rtrim($_GET['url'], '/');
 $url = filter_var($url, FILTER_SANITIZE_URL);
 $url = explode('/', $url);
 return $url;
 }
 }
 }

	 

	Alguém poderia dar uma força ai?