toJson method Null safety

Implementation

Map<String, dynamic> toJson() {
  final Map<String, dynamic> data = new Map<String, dynamic>();
  data['userSig'] = this.userSig;
  data['roomId'] = this.roomId;
  data['strRoomId'] = this.strRoomId;
  data['privateMapKey'] = this.privateMapKey;
  return data;
}