code: private static function getErrorCallback()
      {
          $errorCallback = self::$errorCallback;
  
          self::$errorCallback = false;
  
          if (! $errorCallback) {
              return false;
          }
  
          if (! is_object($errorCallback)) {
              return self::invokeObject($errorCallback);
          }
  
          return call_user_func($errorCallback);
      }
docs: Get error callback if route does not exists.
  
  @since 1.0.3
  
  @return callable