resolve JSON data that doesn't show up when called. in listview flutter

I am trying to display API data into a listview builder and when I try to call the data the response is 200 and there is no error.

however, the data does not appear when you want to display it to the user.

Is there anyone here who has had a problem like mine?
if so, how did you handle it?
Thank You.

and this is the function when i call data API

Future<NilaiMahasiswa> getNilaiMahasiswa() async {
    String url = Constant.baseURL;
    String token = await UtilSharedPreferences.getToken();
    final response = await http.get(
      Uri.parse(
        '$url/auth/mhs_siakad/transkrip_nilai',
      ),
      headers: {
        'Authorization': 'Bearer $token',
      },
    );

    print(response.statusCode);
    print(response.body);
    if (response.statusCode == 200) {
      return NilaiMahasiswa.fromJson(jsonDecode(response.body));
    } else {
      throw Exception();
    }
  }

the following is when I display the data to the user

class _NilaiMahasiswaPageState extends State<NilaiMahasiswaPage> {
  List? data = [];

  @override
  void initState() {
    super.initState();
    Services().getNilaiMahasiswa();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: PreferredSize(
        preferredSize: const Size.fromHeight(kToolbarHeight),
        child: CustomAppbar(
          title: 'Nilai Mahasiswa',
        ),
      ),
      body: ListView.builder(
        itemCount: data!.length,
        itemBuilder: (BuildContext context, index) {
          return Padding(
            padding: const EdgeInsets.only(left: 12, right: 12, top: 12),
            child: Container(
              decoration: BoxDecoration(
                color: Colors.white,
                borderRadius: const BorderRadius.all(
                  Radius.circular(8),
                ),
                boxShadow: [
                  BoxShadow(
                    color: Colors.grey.withOpacity(0.2),
                    spreadRadius: 1,
                    blurRadius: 9,
                    offset: const Offset(
                      1,
                      2,
                    ),
                  ),
                ],
              ),
              width: double.infinity,
              child: Padding(
                padding: const EdgeInsets.all(16),
                child: Column(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: [
                    Text(
                      data![index]["id_mk"],
                      style: const TextStyle(fontSize: 16),
                    ),
                    const SizedBox(
                      height: 12,
                    ),
                    Text(
                      data![index]['nm_mk'],
                    ),
                  ],
                ), ...

and this model

class NilaiMahasiswa {
  String? status;
  String? code;
  List<Data>? data;

  NilaiMahasiswa({this.status, this.code, this.data});

  NilaiMahasiswa.fromJson(Map<String, dynamic> json) {
    status = json['status'];
    code = json['code'];
    if (json['data'] != null) {
      data = <Data>[];
      json['data'].forEach((v) {
        data!.add(Data.fromJson(v));
      });
    }
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = <String, dynamic>{};
    data['status'] = status;
    data['code'] = code;
    if (this.data != null) {
      data['data'] = this.data!.map((v) => v.toJson()).toList();
    }
    return data;
  }
}

class Data {
  String? idTranskripNilai;
  String? idMk;
  String? kodeMk;
  String? nmMk;
  int? sks;
  int? smt;
  String? nilaiAkhirUts;
  String? nilaiHurufUts;
  String? nilaiIndeksUts;
  String? nilaiAkhirUas;
  String? nilaiAkhir;
  String? nilaiHurufAkhir;
  String? nilaiIndeksAkhir;
  int? statusNilaiAkhir;
  int? statusNilaiUts;
  String? updatedBy;

  Data(
      {this.idTranskripNilai,
      this.idMk,
      this.kodeMk,
      this.nmMk,
      this.sks,
      this.smt,
      this.nilaiAkhirUts,
      this.nilaiHurufUts,
      this.nilaiIndeksUts,
      this.nilaiAkhirUas,
      this.nilaiAkhir,
      this.nilaiHurufAkhir,
      this.nilaiIndeksAkhir,
      this.statusNilaiAkhir,
      this.statusNilaiUts,
      this.updatedBy});

  Data.fromJson(Map<String, dynamic> json) {
    idTranskripNilai = json['id_transkrip_nilai'];
    idMk = json['id_mk'];
    kodeMk = json['kode_mk'];
    nmMk = json['nm_mk'];
    sks = json['sks'];
    smt = json['smt'];
    nilaiAkhirUts = json['nilai_akhir_uts'];
    nilaiHurufUts = json['nilai_huruf_uts'];
    nilaiIndeksUts = json['nilai_indeks_uts'];
    nilaiAkhirUas = json['nilai_akhir_uas'];
    nilaiAkhir = json['nilai_akhir'];
    nilaiHurufAkhir = json['nilai_huruf_akhir'];
    nilaiIndeksAkhir = json['nilai_indeks_akhir'];
    statusNilaiAkhir = json['status_nilai_akhir'];
    statusNilaiUts = json['status_nilai_uts'];
    updatedBy = json['updated_by'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = <String, dynamic>{};
    data['id_transkrip_nilai'] = this.idTranskripNilai;
    data['id_mk'] = this.idMk;
    data['kode_mk'] = this.kodeMk;
    data['nm_mk'] = this.nmMk;
    data['sks'] = this.sks;
    data['smt'] = this.smt;
    data['nilai_akhir_uts'] = this.nilaiAkhirUts;
    data['nilai_huruf_uts'] = this.nilaiHurufUts;
    data['nilai_indeks_uts'] = this.nilaiIndeksUts;
    data['nilai_akhir_uas'] = this.nilaiAkhirUas;
    data['nilai_akhir'] = this.nilaiAkhir;
    data['nilai_huruf_akhir'] = this.nilaiHurufAkhir;
    data['nilai_indeks_akhir'] = this.nilaiIndeksAkhir;
    data['status_nilai_akhir'] = this.statusNilaiAkhir;
    data['status_nilai_uts'] = this.statusNilaiUts;
    data['updated_by'] = this.updatedBy;
    return data;
  }
}