flutter shared preference null is returned - flutter

Question

everyone
I saved List in shared preference.
Since List cannot be saved in shared preference, it is saved in json type.
I referenced the post on the stackover.
However, when reading the value stored in the preference, null was returned.
Please help.
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:real_lotto/lotto_number.dart';
class DisplayNumber extends StatefulWidget {
#override
_DisplayNumberState createState() => _DisplayNumberState();
}
class _DisplayNumberState extends State<DisplayNumber> {
String _number = '';
//initState called when the widget is mounted
void initState() {
super.initState();
if (_number == '') {
loadLotto().then((String s) => setState((() {
_number = s;
print(s);
})));
}
}
#override
Widget build(BuildContext context) {
print(_number);
return Scaffold();
}
}
Future<void> _saveLotto(number) async {
SharedPreferences prefs = await SharedPreferences.getInstance();
prefs.setString('key', json.encode(number));
}
Future<String> loadLotto() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
var number = json.decode(prefs.getString('key'));
return (number);
}
List<dynamic> myNumber() {
var number = [];
number.add((List.generate(45, (index) => ++index)..shuffle()).sublist(0, 6));
number.add((List.generate(45, (index) => ++index)..shuffle()).sublist(0, 6));
number.add((List.generate(45, (index) => ++index)..shuffle()).sublist(0, 6));
number.add((List.generate(45, (index) => ++index)..shuffle()).sublist(0, 6));
number.add((List.generate(45, (index) => ++index)..shuffle()).sublist(0, 6));
for (var i = 0; i < number.length; i++) {
number[i].sort();
}
_saveLotto(number);
return number;
}