login method Null safety

Implementation

@override
Future login() async {
  _reddit = Reddit.createInstalledFlowInstance(
      clientId: await rootBundle.loadString("assets/draw.ini"),
      userAgent: "soreo",
      redirectUri: Uri.parse("soreo://authorize")
  );
  final Uri url = _reddit.auth.url(["*"], "soreo", compactLogin: true);
  final res = await FlutterWebAuth.authenticate(
      url: url.toString(),
      callbackUrlScheme: "soreo"
  );
  await _reddit.auth.authorize(Uri.parse(res).queryParameters["code"]!);
  FlutterSecureStorage storage = const FlutterSecureStorage();
  storage.write(
      key: "redditCredentials",
      value: _reddit.auth.credentials.toJson()
  );
  _state = AuthenticationStatus.authenticated;
}