dart/flutter jsonDecode on HTTP Request with null exceptions

I'm making HTTP Requests in Flutter with following code to find the nearest city by lat/lng:

import 'package:http/http.dart' as HTTP;
import 'dart:convert';
import 'dart:developer';
import 'package:myApp/models/city.dart';

class API {
  static Future<Map<String, dynamic>> call(Map<String, dynamic> data) async {
    return await HTTP.post(Uri.parse('https://myapp.de/api/'),
      headers: <String, String>{
        "Accept":       "application/json",
        "Content-Type": "application/json; charset=UTF-8",
        "User-Agent":   "myApp/1.0.0 (Android)"
      },
      body: jsonEncode(data),
    ).then((response) {
      try {
        return jsonDecode(response.body) as Map<String, dynamic>;
      } catch(e) {
        log("[API] JSON Exception: $e");
        return <String, dynamic>{};
      }
    }).onError((error, stackTrace) {
        log("[API] $error: $stackTrace");

       return <String, dynamic>{};
    });
  }

  static Future<City> getCity(double longitude, double latitude) async {
    return await call(<String, dynamic>{
      "action":     "city",
      "longitude":  longitude,
      "latitude":   latitude
    }).then((data) {
      return CityResponse.fromJson(data).first();
    });
  }
}

The HTTP Request has following Request & Response: Request

{
    "action":   "city",
    "longitude": 8.5122942,
    "latitude": 50.5141822
}

Response

{
    "status": true,
    "result": [
        {
            "name": "Reiskirchen",
            "country": "DE",
            "longitude": "8.51046",
            "latitude": "50.50404",
            "distance": 0.704886732081551
        }
    ]
}

And here is the city class:

class City {
  final String name;
  final String country;
  final String longitude;
  final String latitude;
  final double distance;

  City(
    this.name,
    this.country,
    this.longitude,
    this.latitude,
    this.distance
  );

  City.fromJson(Map<String, dynamic> json) :
    name      = json['name']! as String,
    country   = json['country']! as String,
    longitude = json['longitude']! as String,
    latitude  = json['latitude']! as String,
    distance  = json['distance']! as double;

  Map<String, dynamic> toJson() => {
    'name':       name,
    'country':    country,
    'longitude':  longitude,
    'latitude':   latitude,
    'distance':   distance,
  };
}

class CityResponse {
  final bool status;
  final List<City> result;

  CityResponse(
    this.status,
    this.result,
  );

  CityResponse.fromJson(Map<String, dynamic> json) :
    status  = json['status'] as bool,
    result  = (json['result'] as List).map((entry) => City.fromJson(entry)).toList();

  Map<String, dynamic> toJson() => {
    'status': status,
    'result': (result).map((entry) => entry.toJson()).toList()
  };

  City first() {
    return result[0];
  }
}

An example call will be inited by:

await API.getCity(data.longitude!, data.latitude!);

The problem I have is that I keep getting an indeterminate exception:

I had already tried to add explicit null-ignore-check by using ? or !like json['status']! as bool

How can I fix the problem?

In my opinion, it may also be related to the server, which unexpectedly delivers disconnects (ERR_CONNECTION_TIMED_OUT) because the hoster dreamhost.com is poorly connected to Germany.

How can I avoid this behavior by checking?