flutter - 颤振错误：（动态）=> Null 不是（字符串，动态）的子类型

Question

这是我得到的错误，

[ERROR:flutter/lib/ui/ui_dart_state.cc(199)] 
Unhandled Exception: type '(dynamic) => Null' is not a 
          subtype of type '(String, dynamic) => void' of 'f'
The getter 'vendorDocList' was called on null.
Receiver: null
Tried calling: vendorDocList

代码：

class VendorsDocument {
  String lead_id;
  Future<getVendorsDoc> getVendorDocList(lead_id) async {
    final response  = await http.post(Uri.parse('example.com/vendor_doc'),
        headers:{
          'Accept': 'application/json',
          HttpHeaders.authorizationHeader: 'Bearer $accessToken}',
        },
        body: {'lead_id':'$lead_id'});
    if (response.statusCode == 200) {
      // print(response.body);
      return getVendorsDoc.fromJson( json.decode(response.body) );
    } else {
      throw Exception('No records found');
    }
  }

}
class getVendorsDoc {
  bool status;
  String msg;
  List<VendorDocList> vendorDocList;

  getVendorsDoc({this.status, this.msg, this.vendorDocList});

  getVendorsDoc.fromJson(Map<String, dynamic> json) {
    status = json['status'];
    msg = json['message'];
    vendorDocList = new List<VendorDocList>();
    json['data'].forEach((v) {
      vendorDocList.add(new VendorDocList.fromJson(v));
    });
  }

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

class VendorDocList {
  String logo;

  VendorDocList(
      {this.logo});

  VendorDocList.fromJson(Map<String, dynamic> json) {
    logo = json['logo'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['logo'] = this.logo;
  }
}

Answer 1

Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['logo'] = this.logo;
  }

检查 VendorDocList 类中的这个 toJson 方法，您只是将徽标分配给 data['logo'] 而不返回任何内容，这就是映射函数返回 null 的原因。

你应该像这样返回数据：

Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['logo'] = this.logo;
return data;
  }