_TypeError - 类型“Null”不是“String”类型的子类型； - 颤振

Question

我收到这个错误：

类型“String”不是 get 方法 flutter 中类型“Null”的子类型

来自后端的 get 方法 flutter API 响应

{
    "ticketId": "5ea260c9-0b70-482d-b8ca-7f811a96bd29",
    "ticketNumber": 1524,
    "subject": "Subjet 112",
    "priority": "Low",
    "description": "Postman api post",
    "anyAttachment": false,
    "status": "Open",
    "createdAt": "2022-03-20T05:08:37.672Z",
    "requesterName": "Ayana Mohan",
    "requesterUUID": "90bf9452-b690-47f2-aa0c-4b3809fdb023",
    "requesterPhone": "1231423123",
    "requesterEmail": "[email protected]",
    "customerId": "c8595a40-ea63-4883-9f94-e79f2545dee3",
    "ticketType": "Incident",
    "categoryId": null,
    "category": null,
    "assigneeUUID": null,
    "assigneeName": null,
    "assigneeEmail": null,
    "assigneePhone": null,
    "groupUUID": null,
    "groupName": null,
    "rcaTitle": null,
    "rca": null,
    "rcaSolution": null,
    "rcaWorkaround": null,
    "rcsSymptoms": null,
    "rcaBy": null,
    "rcaDate": null,
    "rcaTag": [],
    "formControl": [],
    "classified": null,
    "initiatedBy": null,
    "initiationComment": null,
    "impact": null,
    "risk": null,
    "downtimeStart": null,
    "downtimeEnd": null,
    "assetName": null,
    "serviceName": null,
    "applicationName": null,
    "submissionStatus": null,
    "planningStatus": null,
    "implementationStatus": null,
    "reviewStatus": null,
    "closeStatus": null,
    "finalStatus": null,
    "ticketView": null,
    "completeDate": null,
    "cabRequired": null,
    "rolloutPlan": null,
    "backoutPlan": null,
    "impactAnalysis": null,
    "reason": null,
    "workflowStatus": null,
    "releasePlan": null,
    "testComment": null,
    "testResult": null,
    "closeComment": null,
    "closeResult": null,
    "refTicketId": null,
    "tenantId": "8e963a4d-75d3-4727-b92d-44f24ef80aad"
}

：这是我使用的 get API 函数

 Future<List<UserTicketResponseModel>> getAllUserIssueTicket() async {
    List<UserTicketResponseModel> userTicketModel = [];
    final _token = await getToken();

    try {
      http.Response response = await http.get(
        Uri.parse(ticketingBaseUrl + USER_TICKET),
        headers: {
          "Content-Type": "application/json",
          "Authorization": "Bearer $_token",
        },
      );

      userTicketLoading = true;
      var userTickets = jsonDecode(response.body);
      userTicketModel = userTickets
          .map((tickets) => UserTicketResponseModel.fromJson(tickets))
          .toList();
      notifyListeners();
      return userTicketModel;
    } on SocketException {
      throw 'No Internet Connection';
    }
  }

模型类：

import 'dart:convert';

UserTicketResponseModel userTicketResponseModelFromJson(String str) =>
    UserTicketResponseModel.fromJson(json.decode(str));

String userTicketResponseModelToJson(UserTicketResponseModel data) =>
    json.encode(data.toJson());

class UserTicketResponseModel {
  UserTicketResponseModel({
    required this.ticketId,
    required this.ticketNumber,
    required this.subject,
    required this.priority,
    required this.description,
    required this.anyAttachment,
    required this.status,
    required this.createdAt,
    required this.requesterName,
    required this.requesterUuid,
    required this.requesterPhone,
    required this.requesterEmail,
    required this.customerId,
    required this.ticketType,
    required this.categoryId,
    required this.category,
    required this.assigneeUuid,
    required this.assigneeName,
    required this.assigneeEmail,
    required this.assigneePhone,
    required this.groupUuid,
    required this.groupName,
    required this.rcaTitle,
    required this.rca,
    required this.rcaSolution,
    required this.rcaWorkaround,
    required this.rcsSymptoms,
    required this.rcaBy,
    required this.rcaDate,
    required this.rcaTag,
    required this.formControl,
    required this.classified,
    required this.initiatedBy,
    required this.initiationComment,
    required this.impact,
    required this.risk,
    required this.downtimeStart,
    required this.downtimeEnd,
    required this.assetName,
    required this.serviceName,
    required this.applicationName,
    required this.submissionStatus,
    required this.planningStatus,
    required this.implementationStatus,
    required this.reviewStatus,
    required this.closeStatus,
    required this.finalStatus,
    required this.ticketView,
    required this.completeDate,
    required this.cabRequired,
    required this.rolloutPlan,
    required this.backoutPlan,
    required this.impactAnalysis,
    required this.reason,
    required this.workflowStatus,
    required this.releasePlan,
    required this.testComment,
    required this.testResult,
    required this.closeComment,
    required this.closeResult,
    required this.refTicketId,
    required this.tenantId,
  });

  String ticketId;
  int ticketNumber;
  String subject;
  String priority;
  String description;
  bool anyAttachment;
  String status;
  DateTime createdAt;
  String requesterName;
  String requesterUuid;
  String requesterPhone;
  String requesterEmail;
  String customerId;
  String ticketType;
  dynamic categoryId;
  dynamic category;
  dynamic assigneeUuid;
  dynamic assigneeName;
  dynamic assigneeEmail;
  dynamic assigneePhone;
  dynamic groupUuid;
  dynamic groupName;
  dynamic rcaTitle;
  dynamic rca;
  dynamic rcaSolution;
  dynamic rcaWorkaround;
  dynamic rcsSymptoms;
  dynamic rcaBy;
  dynamic rcaDate;
  List<dynamic> rcaTag;
  dynamic formControl;
  dynamic classified;
  dynamic initiatedBy;
  dynamic initiationComment;
  dynamic impact;
  dynamic risk;
  dynamic downtimeStart;
  dynamic downtimeEnd;
  dynamic assetName;
  dynamic serviceName;
  dynamic applicationName;
  dynamic submissionStatus;
  dynamic planningStatus;
  dynamic implementationStatus;
  dynamic reviewStatus;
  dynamic closeStatus;
  dynamic finalStatus;
  dynamic ticketView;
  dynamic completeDate;
  dynamic cabRequired;
  dynamic rolloutPlan;
  dynamic backoutPlan;
  dynamic impactAnalysis;
  dynamic reason;
  dynamic workflowStatus;
  dynamic releasePlan;
  dynamic testComment;
  dynamic testResult;
  dynamic closeComment;
  dynamic closeResult;
  dynamic refTicketId;
  String tenantId;

  factory UserTicketResponseModel.fromJson(Map<String, dynamic> json) =>
      UserTicketResponseModel(
        ticketId: json["ticketId"],
        ticketNumber: json["ticketNumber"],
        subject: json["subject"],
        priority: json["priority"],
        description: json["description"],
        anyAttachment: json["anyAttachment"],
        status: json["status"],
        createdAt: DateTime.parse(json["createdAt"]),
        requesterName: json["requesterName"],
        requesterUuid: json["requesterUUID"],
        requesterPhone: json["requesterPhone"],
        requesterEmail: json["requesterEmail"],
        customerId: json["customerId"],
        ticketType: json["ticketType"],
        categoryId: json["categoryId"],
        category: json["category"],
        assigneeUuid: json["assigneeUUID"],
        assigneeName: json["assigneeName"],
        assigneeEmail: json["assigneeEmail"],
        assigneePhone: json["assigneePhone"],
        groupUuid: json["groupUUID"],
        groupName: json["groupName"],
        rcaTitle: json["rcaTitle"],
        rca: json["rca"],
        rcaSolution: json["rcaSolution"],
        rcaWorkaround: json["rcaWorkaround"],
        rcsSymptoms: json["rcsSymptoms"],
        rcaBy: json["rcaBy"],
        rcaDate: json["rcaDate"],
        rcaTag: List<dynamic>.from(json["rcaTag"].map((x) => x)),
        formControl: json["formControl"],
        classified: json["classified"],
        initiatedBy: json["initiatedBy"],
        initiationComment: json["initiationComment"],
        impact: json["impact"],
        risk: json["risk"],
        downtimeStart: json["downtimeStart"],
        downtimeEnd: json["downtimeEnd"],
        assetName: json["assetName"],
        serviceName: json["serviceName"],
        applicationName: json["applicationName"],
        submissionStatus: json["submissionStatus"],
        planningStatus: json["planningStatus"],
        implementationStatus: json["implementationStatus"],
        reviewStatus: json["reviewStatus"],
        closeStatus: json["closeStatus"],
        finalStatus: json["finalStatus"],
        ticketView: json["ticketView"],
        completeDate: json["completeDate"],
        cabRequired: json["cabRequired"],
        rolloutPlan: json["rolloutPlan"],
        backoutPlan: json["backoutPlan"],
        impactAnalysis: json["impactAnalysis"],
        reason: json["reason"],
        workflowStatus: json["workflowStatus"],
        releasePlan: json["releasePlan"],
        testComment: json["testComment"],
        testResult: json["testResult"],
        closeComment: json["closeComment"],
        closeResult: json["closeResult"],
        refTicketId: json["refTicketId"],
        tenantId: json["tenantId"],
      );

  Map<String, dynamic> toJson() => {
        "ticketId": ticketId,
        "ticketNumber": ticketNumber,
        "subject": subject,
        "priority": priority,
        "description": description,
        "anyAttachment": anyAttachment,
        "status": status,
        "createdAt": createdAt.toIso8601String(),
        "requesterName": requesterName,
        "requesterUUID": requesterUuid,
        "requesterPhone": requesterPhone,
        "requesterEmail": requesterEmail,
        "customerId": customerId,
        "ticketType": ticketType,
        "categoryId": categoryId,
        "category": category,
        "assigneeUUID": assigneeUuid,
        "assigneeName": assigneeName,
        "assigneeEmail": assigneeEmail,
        "assigneePhone": assigneePhone,
        "groupUUID": groupUuid,
        "groupName": groupName,
        "rcaTitle": rcaTitle,
        "rca": rca,
        "rcaSolution": rcaSolution,
        "rcaWorkaround": rcaWorkaround,
        "rcsSymptoms": rcsSymptoms,
        "rcaBy": rcaBy,
        "rcaDate": rcaDate,
        "rcaTag": List<dynamic>.from(rcaTag.map((x) => x)),
        "formControl": formControl,
        "classified": classified,
        "initiatedBy": initiatedBy,
        "initiationComment": initiationComment,
        "impact": impact,
        "risk": risk,
        "downtimeStart": downtimeStart,
        "downtimeEnd": downtimeEnd,
        "assetName": assetName,
        "serviceName": serviceName,
        "applicationName": applicationName,
        "submissionStatus": submissionStatus,
        "planningStatus": planningStatus,
        "implementationStatus": implementationStatus,
        "reviewStatus": reviewStatus,
        "closeStatus": closeStatus,
        "finalStatus": finalStatus,
        "ticketView": ticketView,
        "completeDate": completeDate,
        "cabRequired": cabRequired,
        "rolloutPlan": rolloutPlan,
        "backoutPlan": backoutPlan,
        "impactAnalysis": impactAnalysis,
        "reason": reason,
        "workflowStatus": workflowStatus,
        "releasePlan": releasePlan,
        "testComment": testComment,
        "testResult": testResult,
        "closeComment": closeComment,
        "closeResult": closeResult,
        "refTicketId": refTicketId,
        "tenantId": tenantId,
      };
}

这是我的监听函数：

List<UserTicketResponseModel> _userTicketModel = [];
  @override
  void initState() {
    super.initState();
    getUserTickets();
}

Future<void> getUserTickets() async {
    UserTicketProvider provider =
        Provider.of<UserTicketProvider>(context, listen: false);
    _userTicketModel = await provider.getAllUserIssueTicket();
    print("User Tickets Main: " + _userTicketModel.toString());
  }


body: ListView.builder(
        itemCount: _userTicketModel.length,
        itemBuilder: (context, index) {
          return SingleChildScrollView(
            child: UserTicketsList(
              ticketNumber: _userTicketModel[index].ticketNumber.toString(),
              requester: _userTicketModel[index].requesterName.toString(),
             
              subject: _userTicketModel[index].subject.toString(),
              priorityType: _userTicketModel[index].priority.toString(),
              statusType: _userTicketModel[index].status.toString(),
            ),
          );
        },
      ),

当我运行时应用程序在模型类中发生异常并显示此错误：

_TypeError 类型“Null”不是类型“String”的子类型

我是初学者，我不明白问题是什么！我尝试使用 ?? 或 String? 进行模型类 null 检查，但不起作用！