Why not work

geojson.dart

import 'dart:convert';

void main() {
  final data = {
    "data": {
      "statistics": {
        "activitiesMap": [
          {
            "startTime": "2025-01-02T07:11:09+00:00",
            "location": {"lat": 0, "lon": 0},
            "totalBales": 78
          },
          {
            "startTime": "2025-01-03T07:02:32+00:00",
            "location": {"lat": 0, "lon": 0},
            "totalBales": 162
          },
          {
            "startTime": "2025-01-06T07:07:33+00:00",
            "location": {"lat": 0, "lon": 0},
            "totalBales": 51
          },
          {
            "startTime": "2025-01-06T11:27:45+00:00",
            "location": {"lat": 0, "lon": 0},
            "totalBales": 180
          },
          {
            "startTime": "2025-01-07T09:51:43+00:00",
            "location": {"lat": 0, "lon": 0},
            "totalBales": 261
          },
          {
            "startTime": "2025-01-08T11:32:19+00:00",
            "location": {"lat": 0, "lon": 9.819928169250488},
            "totalBales": 180
          },
          {
            "startTime": "2025-01-09T07:33:01+00:00",
            "location": {"lat": 0, "lon": 0},
            "totalBales": 401
          },
          {
            "startTime": "2025-01-13T09:30:06+00:00",
            "location": {"lat": 0, "lon": 0},
            "totalBales": 333
          },
          {
            "startTime": "2025-01-14T07:21:42+00:00",
            "location": {"lat": 0, "lon": 0},
            "totalBales": 349
          },
          {
            "startTime": "2025-01-15T10:09:07+00:00",
            "location": {"lat": 0, "lon": 0},
            "totalBales": 29
          },
          {
            "startTime": "2025-01-16T11:49:55+00:00",
            "location": {"lat": 0, "lon": 0},
            "totalBales": 64
          },
          {
            "startTime": "2025-01-17T11:15:11+00:00",
            "location": {"lat": 0, "lon": 0},
            "totalBales": 1
          }
        ]
      }
    }
  };

  final activities =
      data["data"]?["statistics"]?["activitiesMap"] as List? ?? [];
  final geoActivities = activities
      .map(
        (activity) => (
          {
            "type": "Feature",
            "properties": {
              "id": activity?["activityId"]?.toString() ?? "",
              "time": DateTime.tryParse(activity?["startTime"] ?? "")
                  ?.millisecondsSinceEpoch,
              "mag": activity?["totalBales"],
            },
            "geometry": {
              "type": "Point",
              "coordinates": [
                activity?["location"]?["lat"] ?? 0,
                activity?["location"]?["lon"] ?? 0,
              ],
            },
          },
        ),
      )
      .toList();

  final geoJson = {
    "type": "FeatureCollection",
    "crs": {
      "type": "name",
      "properties": {"name": "urn:ogc:def:crs:OGC:1.3:CRS84"},
    },
    "features": geoActivities,
  };

  print(geoJson);

  final geoString = json.encode(geoJson);

  print(geoString);
}