toMap method Null safety

Implementation

@override
Map<String, Object?> toMap() {
  return {
    'startPoiList': this.startPoiList?.map((e) => e.toMap()).toList(),
    'startCityList': this.startCityList?.map((e) => e.toMap()).toList(),
    'endPoiList': this.endPoiList?.map((e) => e.toMap()).toList(),
    'endCityList': this.endCityList?.map((e) => e.toMap()).toList(),
    'wayPointPoiList': this
        .wayPointPoiList
        ?.map((list) => list.map((info) => info.toMap()).toList())
        .toList(),
    'wayPointCityList': this
        .wayPointCityList
        ?.map((list) => list.map((cityInfo) => cityInfo.toMap()).toList())
        .toList()
  };
}