import 'package:flutter/material.dart';

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

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: HomeScreen(title: 'Flutter Demo Home Page'),
    );
  }
}

class HomeScreen extends StatefulWidget {
  HomeScreen({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _HomeScreenState createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  TextEditingController controller = TextEditingController();

  bool isEmpty = true;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: Container(
          padding: EdgeInsets.all(16),
          child: TextField(
            keyboardType: TextInputType.multiline,
            maxLines: null,
            controller: controller,
            onChanged: (value) {
              setState(() {
                // Toggle this for expected behavior
                // isEmpty = value.isEmpty;
              });
            },
            decoration: InputDecoration(
                labelText: 'Title',
                hintMaxLines: isEmpty ? 15 : 1,
                hintText:
                  Iterable.generate(400, (index) => 'a').join(),
                    errorText: 'This is an error'),
          ),
        ));
  }
}

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, v1.12.13+hotfix.8, on Mac OS X 10.15.3 19D76, locale en-DE)
 
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
[✓] Xcode - develop for iOS and macOS (Xcode 11.3.1)
[✓] Android Studio (version 3.5)
[!] IntelliJ IDEA Ultimate Edition (version 2019.3)
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
[✓] VS Code (version 1.42.1)
[✓] Connected device (1 available)

! Doctor found issues in 1 category.