guid-empty

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
    );

guid-empty

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
    );

guid-empty

import 'package:flutter/material.dart';
import 'dart:math';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),

guid-empty

import 'package:flutter/material.dart';
import 'dart:math';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),

guid-empty

// Copyright 2019 The Flutter team. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());

guid-empty

import 'package:flutter/material.dart';
// Copyright 2019-present the Flutter authors. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software

guid-empty

Попробуйте задать явным образом тип коллбек-функции с использованием typedef.
Во вкладке solution мы можете увидеть решение проблемы с неявно заданной сигнатурой коллбек функции. 

guid-empty

Подумайте, как можно было бы упростить метод build()? 
Попробуйте самостоятельно или посмотрите solution

guid-empty

/// /// Flutter Mobile Developer /// "Conditional member access" or "null safe" operator /// Still actual but keep in the mind /// https://nullsafety.dartpad.dev/ ///

class Address { String postalCode; }

guid-empty

///
/// Flutter Mobile Developer
/// Strings
/// https://api.dart.dev/stable/2.10.1/dart-core/String-class.html

void main() {
  
  // что насчет объявления строк?
  final currentDateString = 'Current Date is ' + DateTime.now().toString();
  print(currentDateString);