Navigation on Simple Auth with Flutter

On https://codewithandrea.com/articles/2019-06-03-simple-authentication-flow-with-flutter/

could you just as simply do

if (user == null) {
return Navigator.of(context).push("/login");
}

instead of

if (user == null) {
return SignInPage();
}

and, how do you add a "router guard" on each of your routes to check sign in?