Response to SO question: https://stackoverflow.com/questions/53491536/how-to-navigate-to-another-page-on-load-in-flutter

main.dart

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());


class MyApp extends StatelessWidget {

    /// Use these booleans to see how each approach works, use the one that fits your needs
    /// *** For some strange reason HotReload is not responding to changes in these bools
    /// but if you use HotRestart then it's fine.
    ///
    /// Are you going to be using the setting button?
    final bool useSettingButton = true;
    /// Is the user already logged in?
    final bool userIsLoggedIn = true;

    @override
    Widget build(BuildContext context) {
        return MaterialApp(
            home: useSettingButton == true ?
            UsingSettingsButton(userIsLoggedIn: userIsLoggedIn) :
            DoesNotUseSettingButton(loggedIn: userIsLoggedIn),
        );
    }
}

class DoesNotUseSettingButton extends StatelessWidget {
    final bool loggedIn;

    DoesNotUseSettingButton({
        @required this.loggedIn,
    });

    @override
    Widget build(BuildContext context) {
        Widget result;
        if (loggedIn == true) {
            result = LoggedIn();
        } else {
            result = NotLoggedIn();
        }
        return result;
    }
}

class UsingSettingsButton extends StatelessWidget {
    const UsingSettingsButton({
        Key key,
        @required this.userIsLoggedIn,
    }) : super(key: key);

    final bool userIsLoggedIn;

    @override
    Widget build(BuildContext context) {
        return Scaffold(
            appBar: AppBar(
                title: Text('Rera Farm'),
                actions: <Widget>[
                    PopupMenuButton(
                        itemBuilder: (BuildContext context) {
                            return <PopupMenuEntry>[
                                PopupMenuItem(
                                    child: ListTile(
                                        title: Text('Settings'),
                                        onTap: () {
                                            Navigator.push(context, MaterialPageRoute(
                                                builder: (BuildContext context) =>
                                                userIsLoggedIn == true ?
                                                LoggedIn() :
                                                NotLoggedIn(),
                                            ),
                                            );
                                        },
                                    ),
                                ),
                            ];
                        },
                    )
                ],
            ),
            body: Container(
                height: double.infinity,
                width: double.infinity,
                color: Colors.blue,
            ),
        );
    }
}

class LoggedIn extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
        return Container(
            height: double.infinity,
            width: double.infinity,
            color: Colors.green,
        );
    }
}

class NotLoggedIn extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
        return Container(
            height: double.infinity,
            width: double.infinity,
            color: Colors.red,
        );
    }
}