Getting Null Exception while storing data to local storage - dart

Question

I tried to follow the following dart read/write tutorial and stackoverflow answer to create a dart file for reading and writing data to local storage.
Here's my class:
import 'dart:async';
import 'dart:io';
import 'package:path_provider/path_provider.dart';
class FavoritesStorage {
Future<String> get _localPath async {
final directory = await getApplicationDocumentsDirectory();
return directory.path;
}
Future<File> get _localFile async {
final path = await _localPath;
return File('$path/favorites.txt');
}
Future<List<String>> readFavoriteList() async {
try {
final file = await _localFile;
// Read the file
List<String> contents = await file.readAsLines();
return contents;
} catch (e) {
// If we encounter an error, return null
return null;
}
}
Future<File> writeFavoriteList(String favorite) async {
final file = await _localFile;
print("local file" + file.toString());
// Write the file
return file.writeAsString('$favorite');
}
}
Then I am calling the write method in another class like so:
class FavoriteWidget extends StatefulWidget {
final dish_name;
#override
_FavoriteWidgetState createState() => _FavoriteWidgetState();
final FavoritesStorage storage;
FavoriteWidget(this.dish_name, {Key key, #required this.storage})
: super(key: key);
}
class _FavoriteWidgetState extends State<FavoriteWidget> {
bool _isFavorited = false;
// #docregion _toggleFavorite
void _toggleFavorite() {
setState(() {
if (_isFavorited) {
_isFavorited = false;
} else {
//Add to local storage
widget.storage.writeFavoriteList(widget.dish_name);
_isFavorited = true;
}
});
}
#override
Widget build(BuildContext context) {
return Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: [
IconButton(
icon: (_isFavorited
? Icon(Icons.favorite, size: 35, color: Colors.green)
: Icon(Icons.favorite_border, size: 35, color: Colors.green)),
color: Colors.red[500],
onPressed: _toggleFavorite,
),
Container(
margin: const EdgeInsets.only(top: 8.0),
child: Text(
"FAVORITE",
style: TextStyle(
fontSize: 12.0,
fontWeight: FontWeight.w400,
color: Colors.white,
),
),
),
],
);
}
}
However, I am getting the following error message:
The method 'writeFavoriteList' was called on null.