Type 'Null' is not a subtype of type 'String' in type cast - Flutter

I am making a mistake somewhere, unfortunately after displaying the results everything should work but it does not.

I have tried in many ways, but unfortunately only CircularProgressIndicator still appears.Everything seems to be properly done but unfortunately something is missing.

Any help is greatly appreciated!

 class ApiService {
 final endPointUrl = "newsapi.org";
final client = http.Client();

  Future<List<Article>> getArticle() async {

final queryParameters = {
  'country': 'us',
  'category': 'business',
  'apiKey': 'd74673ec25dd468d94ab03035fa53ff7'
};

final uri = Uri.https(endPointUrl, '/v2/top-headlines', queryParameters);
final response = await client.get(uri);
Map<String, dynamic> json = jsonDecode(response.body);
print(response.body);
List<dynamic> body = json['articles'];
print(body);
List<Article> articles = body.map((dynamic item) => Article.fromJson(item)).toList();
print(articles);
return articles;

  }
}

And classes:

class Article {
Source source;
 // String author;
  String title;
 String description;
 String url;
 String urlToImage;
 String publishedAt;


   Article(
     {required this.source,
    // required this.author,
    required this.title,
    required this.description,
    required this.url,
    required this.urlToImage,
    required this.publishedAt,
   });

factory Article.fromJson(Map<String, dynamic> json) {
return Article(
  source: Source.fromJson(json['source']),
  // author: json['author'] as String,
  title: json['title'] as String,
  description: json['description'] as String,
  url: json['url'] as String,
  urlToImage: json['urlToImage'] as String,
  publishedAt: json['publishedAt'] as String,
  );
}
 }


class Source {
  // String id;
   String name;

  Source({ required this.name});

  factory Source.fromJson(Map<String, dynamic> json) {
    return Source(
       // id: json['id'] as String,
        name: json['name']as String);
     }
   }

flutter code:

  Widget mainArticles(BuildContext context) {
return FutureBuilder(
  future: client.getArticle(),
  builder: (BuildContext context, AsyncSnapshot<List<Article>> snapshot) {
    if (snapshot.hasData) {
      List<Article> articles = snapshot.data!;
      return ListView.builder(
          itemCount: articles.length,
          itemBuilder: (context, index) => customListTile(articles[index], context)
      );
    }
    return Center(
      child: CircularProgressIndicator(),
    );
  },
);
  }