Notice: Trying to access array offset on value of type null in Core.php on line 18

[ThinkingChimp]

This seemed to fix it, but could be more proper

Add isset() to $url

if (file_exists('../app/controllers/' . ucwords($url[0]) . '.php')) {
// Will set a new controller
$this->currentController = ucwords**(isset($url[0])**);
unset($url[0]);
}

[andjelijastankovic]

Problem solved :)

[kkyucon]

You can also solve it by using the null coalescing operator defaulting to empty string.
Change this line:

if(file_exists('../app/controllers/' . ucwords($url[0]) . '.php')){

To this:

if(file_exists('../app/controllers/' . ucwords($url[0] ?? '') . '.php')){