mapdev\FacebookMessenger\Exceptions\CouldNotCreateMessage::noRecipientDefined PHP Method

    public static function noRecipientDefined()
    {
        return new static('You must define a recipient PSID for the message to be sent to');
    }

 public function checkRecipient()
 {
     if (!isset($this->recipient_id)) {
         throw CouldNotCreateMessage::noRecipientDefined();
     }
 }