initial widget causing error setting providers

1) initialWidget.dart

class AppInitializationWidget extends ConsumerWidget {
  const AppInitializationWidget({super.key, required this.onLoaded});
  final WidgetBuilder onLoaded;

  @override
  Widget build(BuildContext ctx, WidgetRef ref) => CustomFutureBuilder(
        futures: [
          Firebase.initializeApp(),
          DataController.vehicleDB.getVehicleSpecification('type'),
          DataController.vehicleDB.getVehicleSpecification('class'),
          DataController.vehicleDB.getVehicleSpecification('region'),
          DataController.vehicleDB.getVehicleSpecification('transmission'),
          DataController.companyDB.getCompanySpecialties(),
          rootBundle.loadString('assets/map_style.txt'),
          SharedPreferences.getInstance(),
        ],
        loading: _loading(),
        onDone: (st) {
          _setProviderData(ref, st.data);
          return onLoaded(ctx);
        } 
      );

  void _setProviderData(WidgetRef ref, dynamic data) {
    ref.read(vehicleSpecsProvider.notifier).set({
      VehicleType: data[1],
      VehicleClass: data[2],
      VehicleRegion: data[3],
      VehicleTransmission: data[4],
    });
    ref.read(companySpecialtiesProvider.notifier).set(data[5]);
    ref.read(mapStyleProvider.notifier).set(data[6]);
    DataController.prefs = data[7];
  }
}

2) companySpecialtiesProvider.dart

class CompanySpecialtiesNotifier extends Notifier<List<CompanySpecialty>> {
  @override
  List<CompanySpecialty> build() => [];

  void set(List<CompanySpecialty> specialties) => state = specialties;
}

final companySpecialtiesProvider =
    NotifierProvider<CompanySpecialtiesNotifier, List<CompanySpecialty>>(
  () => CompanySpecialtiesNotifier(),
);

3) custom_future_builder.dart

class CustomFutureBuilder extends StatelessWidget {
  final Future? future;
  final List<Future>? futures;
  final Widget? loading;
  final Function(dynamic st) onDone;
  final Widget? error;

  const CustomFutureBuilder({
    this.futures,
    this.future,
    this.loading,
    required this.onDone,
    this.error,
  });

  @override
  Widget build(BuildContext ctx) => FutureBuilder(
        future: (futures == null) ? future! : Future.wait(futures!),
        builder: (ctx, st) {
          switch (st.connectionState) {
            case ConnectionState.waiting:
              return (loading == null)
                  ? Center(child: SpinningLoading())
                  : loading!;
            default:
              if (!st.hasError) {
                return onDone(st);
              } else {
                return (error == null)
                    ? CustomError(body: '${st.error} \n \n ${st.stackTrace}')
                    : error!;
              }
          }
        },
      );
}