https://stackoverflow.com/questions/63833619/flutter-forms-get-the-list-of-fields-in-error

main.dart

import 'package:flutter/material.dart';

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

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

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

class _MyWidgetState extends State<MyWidget> {
  final _formKey = GlobalKey<FormState>();
  final _formValidationManager = FormValidationManager();

  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Column(
        children: [
          TextFormField(
              focusNode: _formValidationManager.getFocusNodeForField('field1'),
              validator:
                  _formValidationManager.wrapValidator('field1', (value) {
                if (value.isEmpty) {
                  return 'Please enter a value';
                }

                return null;
              })),
          TextFormField(
              focusNode: _formValidationManager.getFocusNodeForField('field2'),
              validator:
                  _formValidationManager.wrapValidator('field2', (value) {
                if (value.isEmpty) {
                  return 'Please enter a value';
                }

                return null;
              })),
          ElevatedButton(
              onPressed: () {
                if (!_formKey.currentState.validate()) {
                  _formValidationManager.erroredFields.first.focusNode
                      .requestFocus();
                }
              },
              child: Text('SUBMIT'))
        ],
      ),
    );
  }

  @override
  void dispose() {
    _formValidationManager.dispose();
    super.dispose();
  }
}

class FormValidationManager {
  final _fieldStates = Map<String, FormFieldValidationState>();

  FocusNode getFocusNodeForField(key) {
    _ensureExists(key);

    return _fieldStates[key].focusNode;
  }

  FormFieldValidator<T> wrapValidator<T>(
      String key, FormFieldValidator<T> validator) {
    _ensureExists(key);

    return (input) {
      final result = validator(input);

      _fieldStates[key].hasError = (result?.isNotEmpty ?? false);

      return result;
    };
  }

  List<FormFieldValidationState> get erroredFields => _fieldStates.entries
      .where((s) => s.value.hasError)
      .map((s) => s.value)
      .toList();

  void _ensureExists(String key) {
    _fieldStates[key] ??= FormFieldValidationState(key: key);
  }

  void dispose() {
    _fieldStates.entries.forEach((s) {
      s.value.focusNode.dispose();
    });
  }
}

class FormFieldValidationState {
  final String key;

  bool hasError;
  FocusNode focusNode;

  FormFieldValidationState({@required this.key})
      : hasError = false,
        focusNode = FocusNode();
}