When setState called after await completion, usually there is not state to update. This an attempt to amend the issue.

response_after_stateful_disposal.dart

import 'dart:async';
import 'dart:convert';
import 'dart:math';
import 'dart:html';

import 'package:flutter/material.dart';

Future<String> fetchHttpRes(int id) async {
  final f1 = 'first_name', f2 = 'last_name', timeoutSec = 300;
  final url = 'https://reqres.in/api/users/$id?delay=$id';
  HttpRequest request = await HttpRequest.request(
    url,
    mimeType: 'application/json',
  ).timeout(
    Duration(seconds: timeoutSec),
    onTimeout: () => throw Exception(
      'Server unresponsive after $timeoutSec seconds',
    ),
  );

  if (request.status != 200)
    throw Exception(
      '${request.statusText} Status: ${request.status}.',
    );
  Map<String, dynamic> result = json.decode(request.responseText);
  if (result.isEmpty)
    throw Exception(
      'User id "$id" not found!',
    );
  String val1 = result['data'][f1], val2 = result['data'][f2];
  return 'id: $id, value: $val1 $val2';
}

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

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  Future<String> futureHttpRes;
  bool updating;

  @override
  void initState() {
    super.initState();
    updateHttpRes();
  }

  Future<void> updateHttpRes() async {
    setState(() => updating = true);
    int userId = Random().nextInt(12) + 1;
    try {
      await (futureHttpRes = fetchHttpRes(userId));
    } finally {
      // Yes I know! Another setState after await. 
      // But this, for some reason, doesn't break.
      setState(() => updating = false);
    }
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Fetch Data Example',
      home: Scaffold(
        appBar: AppBar(
          title: Text('Fetch Data Example'),
        ),
        body: Center(
          child: FutureBuilder<String>(
            future: futureHttpRes,
            builder: (context, snapshot) {
              if (snapshot.connectionState == ConnectionState.waiting) {
                return CircularProgressIndicator();
              }
              String msg;
              if (snapshot.hasError) {
                if (snapshot.error is ProgressEvent) {
                  ProgressEvent pe = snapshot.error as ProgressEvent;
                  msg = 'ProgressEvent type: ${pe.type}';
                } else {
                  msg = snapshot.error.toString();
                }
              } else {
                msg = snapshot.data;
              }
              return Text(msg, style: TextStyle(fontSize: 30));
            },
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: updating ? null : updateHttpRes,
          backgroundColor: updating ? Colors.grey : Colors.blue,
          disabledElevation: 0,
          child: Icon(updating ? Icons.hourglass_empty : Icons.refresh),
        ),
      ),
    );
  }
}