toMap method Null safety

Implementation

@override
Map<String, Object?> toMap() {
  return {
    'country': this.country,
    'province': this.province,
    'city': this.city,
    'district': this.district,
    'town': this.town,
    'streetName': this.streetName,
    'adCode': this.adCode,
    'countryCode': this.countryCode,
    'direction': this.direction,
    'distance': this.distance
  };
}