Sets a default controller for a route if one was not specified.
Symfony\Component\HttpKernel\Event\GetResponseEvent $event: Event that is created to create a response for a request.
public function onRequestSetController(GetResponseEvent $event) {
$request = $event
->getRequest();
if (!$request->attributes
->has('_controller') && $request->attributes
->has('_content')) {
$request->attributes
->set('_controller', '\\Drupal\\Core\\HtmlPageController::content');
}
}