The handler for the route is invalid

I am trying to create a custom REST API endpoint in WordPress using Classes. I've also done the same the traditional way - which worked just fine. However, using classes I am getting an error The handler for the route is invalid.

The code:

class CSS_Ads {

    var $url;
    var $endpointPrefix;
    var $endpointName;

    public function __construct()
    {
        add_action('rest_api_init', array( $this, 'ads_api_route' ) );
    }

    public function ads_api_route() {
        register_rest_route( $this->endpointPrefix, $this->endpointName,
            array(
                'methods'  => 'GET',
                'callback' => 'get_all_ads_api_endpoint'
            )
        );
    }

    public function get_all_ads_api_endpoint($params) {
        // doing my post query and stuff
    }

}

Setting:

$ads = new CSS_Ads();
$ads->url = get_site_url();
$ads->endpointPrefix = 'bs/v1';
$ads->endpointName = 'ads';

Full error:

{"code":"rest_invalid_handler","message":"The handler for the route is invalid","data":{"status":500}}

The query is set to -1, there is only one post on the website so it shouldn't matter.