Divers...

main.dart

void main() {
  var book1 = Book("Titre", null);
  var book2 = null;
  
  print("${book1.description}");
  print("${book2?.description}");
  
  var test = book1.description ?? "default";
  print(test);
  
}

class Book {
  final title;
  final description;
  
   const Book(this.title, this.description);
  
  String toString() {
    return '$title, $description';
  }
}