private function getConfig($key, RouteException $e) {
    $scenario = $e->getOperation();
    $default = $this->config->get("default.{$key}");
    $scenario = $this->config->get("routes.{$scenario}.{$key}");
    foreach ([$scenario, $default] as $candidate) {
      if (!empty(trim($candidate))) {
        return $candidate;
      }
    }
    return NULL;
  }