Trying to access array offset on value of type null in php

Question

For school I have to create a MVC framework project but I keep running into this problem and I cant seem to get it working.

( ! ) Notice: Trying to access array offset on value of type null in C:\Users\Jasper\Downloads\MVCFramework-opzet\MVCFramework-master\mvcframework\app\libraries\Core.php on line 18 Call Stack
# Time Memory Function Location 1 0.0004 361640 {main}( )
...\index.php:0 2 0.0004 361992 require_once('C:\Users\Jasper\Downloads\MVCFramework-opzet\MVCFramework-master\mvcframework\app\require.php)
...\index.php:2 3 0.0012 364608 Core->__construct()
...\require.php:10

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The code at the bottom is require.php line 18,
It exists in the project but for some reason it don't want to connect.

// Look in BLL for first value
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]);
}