[core/not-initialized] Firebase has not been correctly initialized.

I got this error after I we set the (dynamic result = await _auth.signInAnon(); in sign_in.dart )
and I add this code to the main.dart to initialize firebase
WidgetsFlutterBinding.ensureInitialized();
await Firebase.initializeApp(
options: FirebaseOptions(apiKey: "XXXX",
appId: "",
messagingSenderId: "",
projectId: "XXXX")
);

and it works , my issue why we should do that, cause in the course it works without adding these lines.