example

main.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. 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';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
        inputDecorationTheme: InputDecorationTheme(
                fillColor: Colors.white,
                filled: true,
                border:  OutlineInputBorder(
                  gapPadding: 6,
                  borderRadius: BorderRadius.circular(8),
                  borderSide: const BorderSide(color: Colors.black, width: 1.0),
                ),
                focusedBorder: OutlineInputBorder(
                  gapPadding: 6,
                  borderRadius: BorderRadius.circular(8),
                  borderSide: const BorderSide(color: Colors.black, width: 1.0),
                ),
                focusedErrorBorder: OutlineInputBorder(
                  gapPadding: 6,
                  borderRadius: BorderRadius.circular(8),
                  borderSide: const BorderSide(color: Colors.black, width: 1.0),
                ),
        )
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;

  const MyHomePage({
    super.key,
    required this.title,
  }) : super();

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            DropdownButtonFormField<int>(
              autovalidateMode: AutovalidateMode.onUserInteraction,
              validator: (value) => value == 1 ? "Error" : null,
              items: const [DropdownMenuItem(value: 1, child: Text("1"))],
              value: 1,
              onChanged: (value) {}
            ),
          ],
        ),
      ),
     
    );
  }
}