toMap method Null safety

Implementation

@override
Map<String, Object?> toMap() {
  return {
    'from': this.from?.toMap(),
    'to': this.to?.toMap(),
    'pageCapacity': this.pageCapacity,
    'pageIndex': this.pageIndex,
    'incityPolicy': this.incityPolicy?.index,
    'intercityPolicy': this.intercityPolicy?.index,
    'intercityTransPolicy': this.intercityTransPolicy?.index
  };
}