public function off(string $event)
    {
        if (false === ($key = array_search($event, $this->getSupportedEvents(), true))) {
            return null;
        }

        if (!isset($this->eventHandlers[$key]) || !($cb = $this->eventHandlers[$key])) {
            return null;
        }

        $this->eventHandlers[$key] = null;

        return $cb;
    }