Unhandled Exception: type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type

I am trying to fetch data from backend and store it in my app. However when I am using method to fetch it i am facing this error Unhandled Exception: type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'StartChapter'. Is my model set up wrong or the fetching function is written wrong? Can someone advice me something? Thanks

Method that fetches data:

  Future<List<Adventures>> getAdventureList() async {
    var response = await getAdventures(apiClient);
    return List<Adventures>.from((response.data["collection"]).map((json) => Adventures.fromJson(json)));
  }

I am trying to fetch data from my backend, example JSON looks like:

{
   "collection":[
      {
         "completed":true,
         "creator_id":"4b94b85f-696e-4682-9534-572d1b2bc47f",
         "creator_nick":"master_of_adventure69",
         "description":"go on a wonderful adventure through the streets of the old town ...",
         "favorite":true,
         "id":"ffcc3fad-8bc2-4ba3-9b8e-59d77b2bff23",
         "name":"The best adventure in the world",
         "start_chapter":{
            "completed":true,
            "description":"at this point you must demonstrate extraordinary intelligence ...",
            "id":"5130dd90-9738-4a26-aeea-10d7d9c905b5",
            "name":"first point in adventure",
            "position":{
               "lat":53.01379,
               "lon":18.598444
            },
            "question":[
               {
                  "chapter_id":"5130dd90-9738-4a26-aeea-10d7d9c905b5",
                  "description":"What has a tail and barks?",
                  "id":"3718f6a9-fff1-4a15-9a24-ae3fa90b4141",
                  "sort":1,
                  "type":"tip/question"
               }
            ],
            "radius":500,
            "ridle":{
               "answer":"42856",
               "id":"ce8545e6-eeca-41c0-9026-9084df5aad13",
               "type":"number_lock_5"
            },
            "tips":[
               {
                  "chapter_id":"5130dd90-9738-4a26-aeea-10d7d9c905b5",
                  "description":"What has a tail and barks?",
                  "id":"3718f6a9-fff1-4a15-9a24-ae3fa90b4141",
                  "sort":1,
                  "type":"tip/question"
               }
            ]
         }
      }
   ],
   "page":1,
   "page_size":50,
   "total_pages":1
}

Model

class Adventure {
  List<Adventures>? collection;
  int page;
  int page_size;
  int total_pages;
  int total_results;

  Adventure({ this.collection, required this.page, required this.page_size, required this.total_pages,
     required this.total_results });

  factory Adventure.fromJson(Map<String, dynamic> json) {
    return Adventure(
      collection: json['collection'].map((item) => Adventures.fromJson(item)).toList(),
      page: json['page'],
      page_size: json['page_size'],
      total_pages: json['total_pages'],
      total_results: json['total_results']
    );
  }
}

class Adventures {
  bool completed;
  String creatorId;
  String creatorNick;
  String? description;
  bool favorite;
  String id;
  String name;
  StartChapter startChapter;

  Adventures(
      {required this.completed,
      required this.creatorId,
      required this.creatorNick,
        this.description,
      required this.favorite,
      required this.id,
      required this.name,
      required this.startChapter});

  factory Adventures.fromJson(Map<String, dynamic> json) {
    return Adventures(
        completed: json['completed'],
        creatorId: json['creator_id'],
        creatorNick: json['creator_nick'],
        description: json['description'],
        favorite: json['favorite'],
        id: json['id'],
        name: json['name'],
        startChapter: json['start_chapter']
    );
  }
}
and so on

>Solution :

If you have a already StartChapter model and ‘fromJson’ method,
try to change like below.

class Adventures {
  bool completed;
  String creatorId;
  String creatorNick;
  String? description;
  bool favorite;
  String id;
  String name;
  StartChapter startChapter;

  Adventures(
      {required this.completed,
      required this.creatorId,
      required this.creatorNick,
        this.description,
      required this.favorite,
      required this.id,
      required this.name,
      required this.startChapter});

  factory Adventures.fromJson(Map<String, dynamic> json) {
    return Adventures(
        completed: json['completed'],
        creatorId: json['creator_id'],
        creatorNick: json['creator_nick'],
        description: json['description'],
        favorite: json['favorite'],
        id: json['id'],
        name: json['name'],
        startChapter: json['start_chapter'] != null ? StartChapter.fromJson(json['start_chapter']) : null
    );
  }
}

If not, define ‘fromJson’ into the StartChapter class.
Or change startChapter’s type from ‘StartChapter’ to ‘Map<String, dynamic>’ class.

class Adventures {
  bool completed;
  String creatorId;
  String creatorNick;
  String? description;
  bool favorite;
  String id;
  String name;
  Map<String, dynamic> startChapter;
...