Tip 'Null' nu este un subtip al "String" în flutter, firebase

Vreau pentru a afișa tatausaha de date, dar de fiecare dată când mă loghez primesc un mesaj de eroare "de tip null nu este un subtip de tip string", chiar dacă nu utilizați șiruri în model și în firebase. poate cineva să mă ajute?

acest lucru este meu home_page.dart

class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);

  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  @override
  void initState() {
    context.read<TataUsahaCubit>().fetchTataUsaha();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    Widget header() {
      return BlocBuilder<AuthCubit, AuthState>(
        builder: (context, state) {
          if (state is AuthSuccess) {
            return Container(
              margin: EdgeInsets.only(
                  right: defaultMargin, left: defaultMargin, top: 30),
              child: Row(
                children: [
                  Expanded(
                    child: Column(
                      crossAxisAlignment: CrossAxisAlignment.start,
                      children: [
                        Text(
                          'Hay,\n${state.user.name}',
                          style: blackTextStyle.copyWith(
                            fontSize: 24,
                            fontWeight: semiBold,
                          ),
                          overflow: TextOverflow.ellipsis,
                        ),
                        Text(
                          'Ingin Melihat Guru kamu?',
                          style: greyTextStyle.copyWith(
                            fontSize: 16,
                            fontWeight: light,
                          ),
                        ),
                      ],
                    ),
                  ),
                ],
              ),
            );
          } else {
            return SizedBox();
          }
        },
      );
    }

    Widget kepalaSekolah() {
      return Container(
        margin: EdgeInsets.only(
          left: defaultMargin,
          right: defaultMargin,
          top: 30,
        ),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            Text(
              'Kepala Sekolah',
              style: blackTextStyle.copyWith(
                fontSize: 18,
                fontWeight: semiBold,
              ),
            ),
            KepalaSekolahTile(
              name: 'Charles Marsel Palasa, S.Pd.',
              imageUrl: 'assets/kepala_sekolah.jpeg',
              jabatan: 'Kepala Sekolah',
            ),
          ],
        ),
      );
    }

    Widget tataUsaha(List<TataUsahaModel> tatausahah) {
      return Container(
        margin: EdgeInsets.only(
          left: defaultMargin,
          right: defaultMargin,
          top: 20,
        ),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            Text(
              'Tata Usaha',
              style: blackTextStyle.copyWith(
                fontSize: 18,
                fontWeight: semiBold,
              ),
            ),
            Column(
              children: tatausahah.map((TataUsahaModel tatausaha) {
                return TataUsahaTile(tatausaha);
              }).toList(),
            ),
          ],
        ),
      );
    }

    return BlocConsumer<TataUsahaCubit, TataUsahaState>(
        listener: (context, state) {
      if (state is TataUsahaFailed) {
        ScaffoldMessenger.of(context).showSnackBar(
          SnackBar(
            backgroundColor: kRedColor,
            content: Text(state.error),
          ),
        );
      }
    }, builder: (context, state) {
      if (state is TataUsahaSuccess) {
        return ListView(
          children: [
            header(),
            kepalaSekolah(),
            tataUsaha(state.tatausahah),
          ],
        );
      }
      return Center(
        child: CircularProgressIndicator(
          color: kPrimaryColor,
        ),
      );
    });
  }
}

tata_usaha_cubit.dart

part 'tata_usaha_state.dart';

class TataUsahaCubit extends Cubit<TataUsahaState> {
  TataUsahaCubit() : super(TataUsahaInitial());
  void fetchTataUsaha() async {
    try {
      emit(TataUsahaLoading());

      List<TataUsahaModel> tatausahah =
          await TataUsahaService().fetchTataUsaha();
      emit(TataUsahaSuccess(tatausahah));
    } catch (e) {
      emit(TataUsahaFailed(e.toString()));
    }
  }
}

tata_usaha_state.dart

part of 'tata_usaha_cubit.dart';

abstract class TataUsahaState extends Equatable {
  const TataUsahaState();

  @override
  List<Object> get props => [];
}

class TataUsahaInitial extends TataUsahaState {}

class TataUsahaLoading extends TataUsahaState {}

class TataUsahaSuccess extends TataUsahaState {
  final List<TataUsahaModel> tatausahah;
  TataUsahaSuccess(this.tatausahah);
  @override
  List<Object> get props => [tatausahah];
}

class TataUsahaFailed extends TataUsahaState {
  final String error;
  TataUsahaFailed(this.error);
  @override
  List<Object> get props => [error];
}

tata_usaha_model.dart

import 'package:equatable/equatable.dart';

class TataUsahaModel extends Equatable {
  final String id;
  final String name;
  final String tahunMasuk;
  final String posisi;
  final String quotes;
  final String imageUrl;

  TataUsahaModel({
    required this.id,
    this.name = '',
    this.tahunMasuk = '',
    this.posisi = '',
    this.quotes = '',
    this.imageUrl = '',
  });

  factory TataUsahaModel.fromJson(String id, Map<String, dynamic> json) =>
      TataUsahaModel(
        id: id,
        name: json['name'],
        tahunMasuk: json['tahunMasuk'],
        posisi: json['posisi'],
        quotes: json['qoutes'],
        imageUrl: json['imageUrl'],
      );

  Map<String, dynamic> toJson() => {
        'id': id,
        'name': name,
        'tahunMasuk': tahunMasuk,
        'posisi': posisi,
        'quotes': quotes,
        'imageUrl': imageUrl,
      };

  @override
  List<Object?> get props => [
        id,
        name,
        tahunMasuk,
        posisi,
        quotes,
        imageUrl,
      ];
}

tata_usaha_service.dart

class TataUsahaService {
  CollectionReference _tatausahaRef =
      FirebaseFirestore.instance.collection('tatausaha');

  Future<List<TataUsahaModel>> fetchTataUsaha() async {
    try {
      QuerySnapshot result = await _tatausahaRef.get();

      List<TataUsahaModel> tatausaha = result.docs.map((e) {
        return TataUsahaModel.fromJson(e.id, e.data() as Map<String, dynamic>);
      }).toList();
      print(tatausaha);
      return tatausaha;
    } catch (e) {
      throw e;
    }
  }
}