how to use rootBundle to load config in flutter - flutter

Question

I am using rootBundle to load config in flutter(2.5.x), this is my code:
Future<void> main() async {
final jsonStr = await rootBundle.loadString("assets/static/emoji-flags.json", cache: false);
}
and I already config the path in pubspec.yaml like this:
flutter:
uses-material-design: true
assets:
- assets/translations/
- assets/static/
but this code running error like this:
E/flutter (14207): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: Null check operator used on a null value
E/flutter (14207): #0 PlatformAssetBundle.load (package:flutter/src/services/asset_bundle.dart:222:39)
E/flutter (14207): #1 AssetBundle.loadString (package:flutter/src/services/asset_bundle.dart:68:33)
E/flutter (14207): #2 CachingAssetBundle.loadString (package:flutter/src/services/asset_bundle.dart:166:18)
E/flutter (14207): #3 main (package:flutter_learn/main.dart:10:36)
E/flutter (14207): #4 _runMainZoned.<anonymous closure>.<anonymous closure> (dart:ui/hooks.dart:145:25)
E/flutter (14207): #5 _rootRun (dart:async/zone.dart:1428:13)
E/flutter (14207): #6 _CustomZone.run (dart:async/zone.dart:1328:19)
E/flutter (14207): #7 _runZoned (dart:async/zone.dart:1863:10)
E/flutter (14207): #8 runZonedGuarded (dart:async/zone.dart:1851:12)
E/flutter (14207): #9 _runMainZoned.<anonymous closure> (dart:ui/hooks.dart:141:5)
E/flutter (14207): #10 _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:283:19)
E/flutter (14207): #11 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)
E/flutter (14207):
what should I do to load this config success? Am I missing something?

Answer 1

This is my environment reader, I map my json file to the data model.
import 'package:flutter/services.dart';
static Future<void> initializeApp(String? env) async {
env = env ?? "development";
final configString = await rootBundle.loadString('assets/config/env.json');
final jsonMap = json.decode(configString);
_config = ConfigModel.fromMap(jsonMap[env]);
}