类类别错误：未处理异常：类型'_InternalLinkedHashMap

  class Category {
  int id;
  String categoryName;
  String seoUrl;


  Category(this.id,this.categoryName,this.seoUrl);

  Category.fromJson(Map json){
    id = json["id"];
    categoryName = json["categoryName"];
    seoUrl = json["seoUrl"];
  }

  Map toJson(){
    return {
      "id": id,
      "categoryName": categoryName,
      "seoUrl": seoUrl
    };
  }
}

班级主屏幕

错误：未处理的异常：类型_InternalLinkedHashMap

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:http_demo/data/api/category_api.dart';
import 'package:http_demo/models/category.dart';

class MainScreen extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return MainScreenState();
  }
}

class MainScreenState extends State {
  List<Category> categories = List<Category>();
  List<Widget> categoryWidgets = List<Widget>();

  @override
  void initState() {
    getCategoriesFromApi();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(
          "Alışveriş Sistemi",
          style: TextStyle(color: Colors.white),
        ),
        backgroundColor: Colors.blueGrey,
        centerTitle: true,
      ),
      body: Padding(
        padding: EdgeInsets.all(10.0),
        child: Column(
          children: <Widget>[
            SingleChildScrollView(
              scrollDirection: Axis.horizontal,
              child: Row(
                children: categoryWidgets,
              ),
            )
          ],
        ),
      ),
    );
  }

  void getCategoriesFromApi() {
    CategoryApi.getCategories().then((response) {
      setState(() {
        Iterable list = json.decode(response.body);
        //categories = list.map((category) => Category.fromJson(category)).toList();
        this.categories = List<Category>.from(list.map((model)=> Category.fromJson(model)));
        getCategoryWidgets();
      });
    });
  }

  List<Widget> getCategoryWidgets() {
    for (int i = 0; i < categories.length; i++) {
      categoryWidgets.add(getCategoryWidget(categories[i]));
    }
    return categoryWidgets;
  }

  Widget getCategoryWidget(Category category) {
    return FlatButton(
      onPressed: null,
      child: Text(
        category.categoryName,
        style: TextStyle(color: Colors.blueGrey),
      ),
      shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.circular(15.0),
        side: BorderSide(color: Colors.lightGreenAccent),
      ),
    );
  }
}

错误：未处理的异常：类型_InternalLinkedHashMap