NoSuchMethodError (NoSuchMethodError: The method 'map' was called on null - flutter

Question

Occurs exception when I get the chapter list.
So how can I solve this problem?
Please help.
Here is my API response.
{
"success": 1,
"chapter": [
{
"chapter_id": "609cb13f497e3",
"chapter_name": "test",
"subject_id": "5e32874c714fa",
"medium_id": "5d15938aa1344",
"standard_id": "5d1594e283e1a",
"material": null,
"textbook": null,
"test_paper": null,
"test_paper_solution": null,
"subject_memory_map": null,
"active": "1"
}
]
}
The model class which I created in chapter_model.dart file.
// To parse this JSON data, do
//
// final chapterBySubjectModel = chapterBySubjectModelFromJson(jsonString);
import 'dart:convert';
ChapterBySubjectModel chapterBySubjectModelFromJson(String str) => ChapterBySubjectModel.fromJson(json.decode(str));
String chapterBySubjectModelToJson(ChapterBySubjectModel data) => json.encode(data.toJson());
class ChapterBySubjectModel {
ChapterBySubjectModel({
required this.success,
required this.chapter,
});
int success;
List<Chapter> chapter;
factory ChapterBySubjectModel.fromJson(Map<String, dynamic> json) => ChapterBySubjectModel(
success: json["success"],
chapter: List<Chapter>.from(json["chapter"].map((x) => Chapter.fromJson(x))),
);
Map<String, dynamic> toJson() => {
"success": success,
"chapter": List<dynamic>.from(chapter.map((x) => x.toJson())),
};
}
class Chapter {
Chapter({
required this.chapterId,
required this.chapterName,
required this.subjectId,
required this.mediumId,
required this.standardId,
this.material,
this.textbook,
this.testPaper,
this.testPaperSolution,
this.subjectMemoryMap,
required this.active,
});
String chapterId;
String chapterName;
String subjectId;
String mediumId;
String standardId;
dynamic material;
dynamic textbook;
dynamic testPaper;
dynamic testPaperSolution;
dynamic subjectMemoryMap;
String active;
factory Chapter.fromJson(Map<String, dynamic> json) => Chapter(
chapterId: json["chapter_id"],
chapterName: json["chapter_name"],
subjectId: json["subject_id"],
mediumId: json["medium_id"],
standardId: json["standard_id"],
material: json["material"],
textbook: json["textbook"],
testPaper: json["test_paper"],
testPaperSolution: json["test_paper_solution"],
subjectMemoryMap: json["subject_memory_map"],
active: json["active"],
);
Map<String, dynamic> toJson() => {
"chapter_id": chapterId,
"chapter_name": chapterName,
"subject_id": subjectId,
"medium_id": mediumId,
"standard_id": standardId,
"material": material,
"textbook": textbook,
"test_paper": testPaper,
"test_paper_solution": testPaperSolution,
"subject_memory_map": subjectMemoryMap,
"active": active,
};
}
Method which i Created in api_manager.dart file.
Future<List<Chapter>> getChapterBySubject() async {
final chapterUrl =
'$baseUrl/subject/get_by_user_plan?user_id=609cab2cd5b6c&order_id=1620889722609cd07a601af469889697609cab2cd5b6c&standard_id=5d1594e283e1a&medium_id=5d15938aa1344';
final response = await http.get(Uri.parse(chapterUrl));
if (response.statusCode == 200) {
final chapterData = chapterBySubjectModelFromJson(response.body);
final List<Chapter> chapters = chapterData.chapter;
print(chapters);
return chapters;
} else {
return <Chapter>[];
}
}
And view as below in chapter_widget.dart file.
class _ChapterWidgetState extends State<ChapterWidget> {
late bool _loading;
var _chapters = <Chapter>[];
#override
void initState() {
super.initState();
_loading = true;
ApiManager().getChapterBySubject().then((chapters) {
setState(() {
_chapters = chapters;
_loading = false;
});
});
}
#override
Widget build(BuildContext context) {
return ListView.builder(
itemCount: null == _chapters ? 0 : _chapters.length,
//itemCount: _chapters.length,
itemBuilder: (context, index) {
Chapter chapter = _chapters[index];
return Container(
padding: EdgeInsets.all(8),
child: Card(
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(20)),
child: ClipRRect(
borderRadius: BorderRadius.circular(20.0),
child: InkWell(
//child: Image.asset("assets/logos/listbackground.png"),
child: Text(chapter.chapterName),
),
),
),
);
});
}
}
It throws an Exception in Model Class in below line.
List<Chapter>.from(json["chapter"].map((x) => Chapter.fromJson(x))),

Answer 1

You set chapter as required but it seems API says it can be null. So, you should convert your parameters from required to nullable like this:
import 'dart:convert';
ChapterBySubjectModel chapterBySubjectModelFromJson(String str) => ChapterBySubjectModel.fromJson(json.decode(str));
String chapterBySubjectModelToJson(ChapterBySubjectModel data) => json.encode(data.toJson());
class ChapterBySubjectModel {
ChapterBySubjectModel({
this.success,
this.chapter,
});
int success;
List<Chapter> chapter;
factory ChapterBySubjectModel.fromJson(Map<String, dynamic> json) => ChapterBySubjectModel(
success: json["success"] == null ? null : json["success"],
chapter: json["chapter"] == null ? null : List<Chapter>.from(json["chapter"].map((x) => Chapter.fromJson(x))),
);
Map<String, dynamic> toJson() => {
"success": success == null ? null : success,
"chapter": chapter == null ? null : List<Chapter>.from(chapter.map((x) => x)),
};
}