getCategoriesJson method Null safety

Implementation

static String getCategoriesJson() {
  return '{"trivia_categories": [{"id": 9,"name": "GeneralKnowledge"},{"id": 10,"name": "Entertainment: Books"},{}"id": 11,"name": "Entertainment: Film"},{"id": 12,"name": "Entertainment: Music"},{"id": 13,"name": "Entertainment: Musicals & Theatres"},{"id": 14,"name": "Entertainment: Television"},{"id": 15,"name": "Entertainment: Video Games"},{"id": 16,"name": "Entertainment: Board Games"},{"id": 17,"name": "Science & Nature"},{"id": 18,"name": "Science: Computers"},{"id": 19,"name": "Science: Mathematics"},{"id": 20,"name": "Mythology"},{"id": 21,"name": "Sports"},{"id": 22,"name": "Geography"},{"id": 23,"name": "History"},{"id": 24,"name": "Politics"},{"id": 25,"name": "Art"},{"id": 26,"name": "Celebrities"},{"id": 27,"name": "Animals"},{"id": 28,"name": "Vehicles"},{"id": 29,"name": "Entertainment: Comics"},{"id": 30,"name": "Science: Gadgets"},{"id": 31,"name": "Entertainment: Japanese Anime & Manga"},{"id": 32,"name": "Entertainment: Cartoon & Animations"}]}';
}