Как я могу зациклить для добавления данных в список в функции флаттера?

Question

Я хочу сделать полигональные карты во флаттере android, используя данные о долготе и широте с сервера, но я пытался использовать цикл point.add, он возвращает

Performing hot reload...                                                
Reloaded 6 of 607 libraries in 1,734ms.
D/EGL_emulation(17903): eglMakeCurrent: 0xdfa1a120: ver 2 0 (tinfo 0xdfa0f640)
D/EGL_emulation(17903): eglMakeCurrent: 0xb68c3d00: ver 1 0 (tinfo 0xc4d5f300)
W/ani_mapping_io(17903): Accessing hidden method Lsun/misc/Unsafe;->putObject(Ljava/lang/Object;JLjava/lang/Object;)V (greylist, linking, allowed)
D/EGL_emulation(17903): eglMakeCurrent: 0xdfa1a120: ver 2 0 (tinfo 0xdfa0f640)
I/Google Maps Android API(17903): Google Play services package version: 19275022
W/ani_mapping_io(17903): Accessing hidden method Lsun/misc/Unsafe;->putObject(Ljava/lang/Object;JLjava/lang/Object;)V (greylist, linking, allowed)
W/ani_mapping_io(17903): Accessing hidden method Lsun/misc/Unsafe;->putObject(Ljava/lang/Object;JLjava/lang/Object;)V (greylist, linking, allowed)
W/ani_mapping_io(17903): Accessing hidden method Lsun/misc/Unsafe;->putLong(Ljava/lang/Object;JJ)V (greylist, linking, allowed)
E/GoogleMapController(17903): Cannot enable MyLocation layer as location permissions are not granted
D/HostConnection(17903): HostConnection::get() New Host Connection established 0xdfa499d0, tid 18433
D/HostConnection(17903): HostComposition ext ANDROID_EMU_CHECKSUM_HELPER_v1 ANDROID_EMU_dma_v1 ANDROID_EMU_direct_mem ANDROID_EMU_host_composition_v1 ANDROID_EMU_host_composition_v2 ANDROID_EMU_vulkan ANDROID_EMU_deferred_vulkan_commands ANDROID_EMU_vulkan_null_optional_strings ANDROID_EMU_vulkan_create_resources_with_requirements ANDROID_EMU_YUV420_888_to_NV21 ANDROID_EMU_YUV_Cache ANDROID_EMU_async_unmap_buffer GL_OES_vertex_array_object GL_KHR_texture_compression_astc_ldr ANDROID_EMU_gles_max_version_2
D/EGL_emulation(17903): eglMakeCurrent: 0xdfa1a120: ver 2 0 (tinfo 0xdfa0f640)
D/EGL_emulation(17903): eglCreateContext: 0xb090f7e0: maj 1 min 0 rcv 1
D/EGL_emulation(17903): eglMakeCurrent: 0xb090f7e0: ver 1 0 (tinfo 0xc4d5f2f0)
E/flutter (17903): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: type 'String' is not a subtype of type 'int'
E/flutter (17903): #0      new DLahan.formJson (package:dutatani_mapping_iot/model/dLahan.dart:17:22)
E/flutter (17903): #1      _DetailLahanState.fetchData.<anonymous closure> (package:dutatani_mapping_iot/detail_lahan.dart:83:34)
E/flutter (17903): #2      State.setState (package:flutter/src/widgets/framework.dart:1148:30)
E/flutter (17903): #3      _DetailLahanState.fetchData (package:dutatani_mapping_iot/detail_lahan.dart:80:7)
E/flutter (17903): <asynchronous suspension>
E/flutter (17903): #4      _DetailLahanState.initState (package:dutatani_mapping_iot/detail_lahan.dart:121:5)
E/flutter (17903): #5      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4355:58)

любое предложение или руководство для цикла данные во флаттере, пожалуйста .. вот мой код

List <DLahan> _listDetail = [];    
 final List<LatLng> points = <LatLng>[];
      Future<Null> fetchData() async{
        _listDetail.clear();
        final response =
        await http.get("<URL>);
        // print(response.body);
        if(response.statusCode ==  200){
          final detailLahan =  jsonDecode(response.body);

          setState(() {
            for (Map i in detailLahan){
              // points.add(_createLatLng(DLahan.formJson(i).lat, DLahan.formJson(i).longt));
              _listDetail.add(DLahan.formJson(i));
            }

          });


          _createPoints();
          _add();
        }
      }

      List<LatLng> _createPoints() {
        final List<LatLng> points = <LatLng>[];


        //not working
        // for (int i=0 ; i<_listDetail.length;i++){
        //   points.add(_createLatLng(_listDetail[i].lat, _listDetail[i].longt));
        // }
        //not working

        //not working
        for(var i in _listDetail){
          points.add(_createLatLng(i.lat, i.longt));
        }
        //not working


        //Manually adding long lat it works but it cant use for me
        // points.add(_createLatLng(-7.------,  1--.----));
        // points.add(_createLatLng(-7.------,  1--.----));
        // points.add(_createLatLng(-7.------,  1--.----));
        // points.add(_createLatLng(-7.------,  1--.----));  
        //Manually adding long lat it works
        return points;
      }

      LatLng _createLatLng(double lat, double lng) {
        return LatLng(lat, lng);
      }

я использовал ручное добавление, точка работает, но это не моя цель, и это мой класс

class DLahan{
  final int id_detail;
  final int id_lahan;
  final double lat;
  final double longt;
  final int indeks;

  DLahan({
    this.id_detail,
    this.id_lahan, 
    this.lat, 
    this.longt,
    this.indeks
  });
  factory DLahan.formJson(Map <String, dynamic> json){
    return new DLahan(
      id_detail: int.parse(json['id_detail']),
      id_lahan:  int.parse(json['ID_Lahan']),
      lat: double.parse(json['lat']),
      longt: double.parse(json['longt']),
      indeks:  int.parse(json['indeks'])
    );
  }

}

извините, если мой неправильная грамматика