toJson method
override
    Object to Map serializer
Implementation
@override
Map<String, dynamic> toJson() {
  return {
    '@type': 'notificationGroup',
    if (extra != null) '@extra': extra,
    'id': id,
    'type': type,
    'chat_id': chat_id,
    'total_count': total_count,
    'notifications': notifications
  };
}