toJson method Null safety

Implementation

Map toJson() {
  Map data = Map();
  data.add("paramType", chatSilentModeParamTypeToInt(_paramType));
  data.add("remindType", chatPushRemindTypeToInt(remindType));
  data.add("startTime", startTime?.toJson());
  data.add("endTime", endTime?.toJson());
  data.add("duration", silentDuration);
  return data;
}