我有一个与API的连接，它是工作与没有问题的 Postman ：例如：

但我对Flutter有个问题：下面是代码：

Future<List<AnswerData>> searchAnswers({required String search}) async {
    final response = await post(
    Uri.parse("$_url/search"),
    headers:  <String, String>{
      'Content-Type': "application/json",
      'Connection' : 'keep-alive',
      'Accept' : '*/*',
    },
    body: jsonEncode(<String, String>{"search" : "oop"})
    );
    print("status code${response.statusCode}");
    final answers = answerFromJson(response.body);
    print("msg error ${answers.data}");
    return answers.data;
  }

返回的状态代码为

(200)

但返回null，并显示以下消息：

I/flutter (10607): status code 200
I/flutter (10607): msg error : type 'Null' is not a subtype of type 'Map<String, dynamic>'

这就是模型：

// To parse this JSON data, do
//
//     final answer = answerFromJson(jsonString);

import 'dart:convert';
import 'dart:io';

Answer answerFromJson(String str) => Answer.fromJson(json.decode(str));

String answerToJson(Answer data) => json.encode(data.toJson());

class Answer {
  Answer({
    required this.data,
    required this.links,
  });

  List<AnswerData> data;
  Links links;

  factory Answer.fromJson(Map<String, dynamic> json) => Answer(
    data:  List<AnswerData>.from(json["data"].map((x) => AnswerData.fromJson(x))),
    links:  Links.fromJson(json["links"]),
  );

  Map<String, dynamic> toJson() => {
    "data": data == null ? null : List<dynamic>.from(data.map((x) => x.toJson())),
    "links": links == null ? null : links.toJson(),
  };
}

class AnswerData {
  AnswerData({
    required this.title,
    required this.text,
    required this.link,
    required this.course,
  });

  String title;
  String text;
  String link;
  String course;

  factory AnswerData.fromJson(Map<String, dynamic> json) => AnswerData(
    title: json["title"] == null ? null : json["title"],
    text: json["text"] == null ? null : json["text"],
    link: json["link"] == null ? null : json["link"],
    course: json["course"] == null ? null : json["course"],
  );

  Map<String, dynamic> toJson() => {
    "title": title == null ? null : title,
    "text": text == null ? null : text,
    "link": link == null ? null : link,
    "course": course == null ? null : course,
  };
}

class Links {
  Links({
    required this.first,
    required this.last,
    this.prev,
    this.next,
  });

  String first;
  String last;
  dynamic prev;
  dynamic next;

  factory Links.fromJson(Map<String, dynamic> json) => Links(
    first: json["first"] == null ? null : json["first"],
    last: json["last"] == null ? null : json["last"],
    prev: json["prev"],
    next: json["next"],
  );

  Map<String, dynamic> toJson() => {
    "first": first == null ? null : first,
    "last": last == null ? null : last,
    "prev": prev,
    "next": next,
  };
}