error.dart

import 'package:flutter/material.dart';

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

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

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key key}) : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {
  bool _error = false;

  void _setError() {
    setState(() {
      _error = !_error;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        children: [
          TextFormField(
              style: TextStyle(color: _error ? Colors.black : Colors.red)),
          TextFormField(
              style: TextStyle(color: _error ? Colors.red : Colors.black)),
          FlatButton(
            child: Text('Click me to reproduce error'),
            onPressed: _setError,
          )
        ],
      ),
    );
  }
}