toJson method Null safety

Implementation

Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = <String, dynamic>{};
    data['category'] = category;
    data['type'] = type;
    data['difficulty'] = difficulty;
    data['question'] = question;
    data['correct_answer'] = correctAnswer;
    data['incorrect_answers'] = incorrectAnswers;
    return data;
}