Using map instead of for statement to make list from class model return zero

when i implementing paginating ListView from data witch that return from server, simply i can test this paginate ability and that's work fine, for example by this code i can make list column by simple data:

list.add(
    PostItem((b) =>b
        ..title = 'lorem ipsum'
        ..colorInt = _randomGenerator.nextInt(0xFFFFFFFF)),
    );

return BuiltList.of(list);

now i'm trying to get data from server and make this data from that like with:

List<PostItem> list = [
  PostItem((b)=>b..title = 'lorem ipsum'),
  PostItem((b)=>b..title = 'lorem ipsum')
];

i get error:

List<PostItem> list=[];

final response = await http.get(Constants.getPosts, headers: {"Accept": "application/json"});

final responseString = json.decode(response.body) as List;

List<ContentsModel> responses = responseString.map((j) => 
    ContentsModel.fromJson(j)).toList();

responses.map((post)=>
    list.add(PostItem((b) => b..title = post.title),));

return BuiltList.of(list);

in this line size of list is zero:

responses.map((post)=>list.add(PostItem((b) => b..title = post.title),));

my ContentsModel class content is:

part 'contents_model.g.dart';

@JsonSerializable(nullable: false)
class ContentsModel{
  int id;
  String title;
  String description;

  ContentsModel(this.id, this.postId, this.title, this.description, this.type, this.featuredImages, this.slug, this.lang, this.visit, this.click, this.state, this.createdAt, this.updatedAt, this.categories);

  factory ContentsModel.fromJson(Map<String,dynamic> json)=>_$ContentsModelFromJson(json);
  Map<String,dynamic> toJson()=>_$ContentsModelToJson(this);
}

You need initialize your list, before add something:

List<PostItem> list = [];