setState() called in constructor, Can I change the state of a widget from another class?

Question

I'm getting this error when I'm trying to change the state of the widget by defining a custom function to set state and then I'm calling that function from a new class I created.

setState() called in constructor: _MyHomePageState#da7cc(lifecycle state: created, no widget, not mounted)

HomePage:

import 'package:flutter/material.dart';
import 'package:ed_cell/auth_service.dart';
import 'package:flutter/scheduler.dart';

Widget authStatus = Text('Welcome');

class MyHomePage extends StatefulWidget {
  _MyHomePageState myHomePageState= new _MyHomePageState();
  @override
  _MyHomePageState createState() => myHomePageState;
}

class _MyHomePageState extends State<MyHomePage> {

  @override
  Widget build(BuildContext context) {
    return ListView(
      children: [
        Container(
          decoration: BoxDecoration(
              color: Colors.white, borderRadius: BorderRadius.circular(10)),
          height: 575,
          child: Row(
            children: <Widget>[
              Column(
                children: [
                  Expanded(
                    child: Image.asset(
                      'assets/images/rocket.png',
                    ),
                  ),
                ],
              ),
              Expanded(
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.spaceAround,
                  children: [
                    Text(
                      'U create\nWe support',
                      style:
                          TextStyle(fontSize: 80, fontWeight: FontWeight.bold),
                    ),
                    authStatus,
                  ],
                ),
              ),
            ],
          ),
        ),
      ],
    );
  }

  void onAuthChange() {
     var x=AuthService().status;
    if (AuthService().status == 'Sucessful Sign In') {
      setState(()  {
        authStatus = Text(
          'Welcome',
          style: TextStyle(color: Colors.grey, fontSize: 50),
        );
        print(x);
      });
      print('object'+x);
    } else {
      setState(()  {
        authStatus = RaisedButton(
          onPressed: null,
          color: Colors.deepOrange,
          child: Text(
            'Join Now',
            style: TextStyle(color: Colors.white, fontWeight: FontWeight.bold),
          ),
        );
      });
    }
  }
}

AuthService:

import 'package:firebase_auth/firebase_auth.dart';
import 'package:ed_cell/home_page.dart';

class AuthService{
  String status;

  signIn(email,password){
    FirebaseAuth.instance.signInWithEmailAndPassword(email: email, password: password).then((user){
      print('Signed In '+user.user.uid);
      status='Sucessful Sign In';
    }).catchError((e){
      status='Failure in Sign In';
      print(e);
    });
    MyHomePage().myHomePageState.onAuthChange();
  }
}

Answer 1

The error relates to HomePage not being inserted into the widget tree yet. You can call setState from another class as you have already done correctly, but the HomePage widget has to be somewhere already in the widget tree for it to be able to do setState.