TextFormFiled Error Border for dart pad

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(
  
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: RegistrationScreen(),
        ),
      ),
    );
  }
}
class RegistrationScreen extends StatefulWidget {
  static const String id = 'registration_screen';
  @override
  _RegistrationScreenState createState() => _RegistrationScreenState();
}

class _RegistrationScreenState extends State<RegistrationScreen> {
  final _formKey = GlobalKey<FormState>();
  bool loading = false;
  String error = '';

  String userName = "";
  String email = "";
  String password = "";
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Padding(
        padding: const EdgeInsets.symmetric(horizontal: 30.0),
        child: Center(
          child: Form(
            key: _formKey,
            child: ListView(
              children: <Widget>[
                Padding(
                  padding: const EdgeInsets.symmetric(vertical: 50.0),
                  child: Center(child: Text('add logo')),
                ),
                Padding(
                  padding: const EdgeInsets.only(left: 20.0, bottom: 10.0),
                  child: Text(
                    'Username:',
                    style: TextStyle(
                      fontSize: 18.0,
                      fontWeight: FontWeight.w600,
                      //fontStyle: FontStyle.italic,
                    ),
                  ),
                ),
                TextFormField(
                  validator: (String str) =>
                      str.isEmpty ? 'Enter a username' : null,
                  decoration: InputDecoration(
                    hintText: 'theChadMaster76',
                    hintStyle: TextStyle(
                      fontStyle: FontStyle.italic,
                    ),
                    enabledBorder: OutlineInputBorder(
                      borderSide: BorderSide(color: Colors.green),
                      borderRadius: BorderRadius.circular(35.0),
                    ),
                    focusedBorder: OutlineInputBorder(

                      borderSide: BorderSide(color: Colors.blueAccent),
                      borderRadius: BorderRadius.circular(35.0),
                    ),
                    errorBorder: OutlineInputBorder(

                      borderSide: BorderSide(color: Colors.red),
                      borderRadius: BorderRadius.circular(35.0),
                    ),
                    prefixIcon: Icon(Icons
                        .account_circle), //make the icon also change its color
                    filled: true,
                    fillColor: Colors.grey.shade200,
                  ),
                  textAlign: TextAlign.center,
                  textAlignVertical: TextAlignVertical.bottom,
                  onChanged: (String str) {
                    setState(() {
                      userName = str;
                    });
                  },
                ),
                SizedBox(height: 30.0),
                Padding(
                  padding: const EdgeInsets.only(left: 20.0, bottom: 10.0),
                  child: Text(
                    'Email:',
                    style: TextStyle(
                      fontSize: 18.0,
                      fontWeight: FontWeight.w600,
                      //fontStyle: FontStyle.italic,
                    ),
                  ),
                ),
                TextFormField(
                  keyboardType: TextInputType.emailAddress,
                  validator: (String str) =>
                      str.isEmpty ? 'Enter an email' : null,
                  decoration: InputDecoration(
                    hintText: 'bobbyBob@gmail.com',
                    hintStyle: TextStyle(
                      fontStyle: FontStyle.italic,
                    ),
                    enabledBorder: OutlineInputBorder(
                      borderSide: BorderSide(color: Colors.green),
                      borderRadius: BorderRadius.circular(35.0),
                    ),
                    focusedBorder: OutlineInputBorder(

                      borderSide: BorderSide(color: Colors.blueAccent),
                      borderRadius: BorderRadius.circular(35.0),
                    ),
                    prefixIcon:
                        Icon(Icons.mail),
                    filled: true,
                    fillColor: Colors.grey.shade200,
                  ),
                  textAlign: TextAlign.center,
                  textAlignVertical: TextAlignVertical.bottom,
                  onChanged: (String str) {
                    setState(() {
                      email = str;
                    });
                  },
                ),
                SizedBox(height: 30.0),
                Padding(
                  padding: const EdgeInsets.only(left: 20.0, bottom: 10.0),
                  child: Text(
                    'Password:',
                    style: TextStyle(
                      fontSize: 18.0,
                      fontWeight: FontWeight.w600,
                      //fontStyle: FontStyle.italic,
                    ),
                  ),
                ),
                TextFormField(
                  validator: (String str) =>
                      str.length < 6 ? 'Enter an password 6+ char' : null,
                  obscureText: true,
                  decoration: InputDecoration(
                    hintText: 'secretPassword123!',
                    hintStyle: TextStyle(
                      fontStyle: FontStyle.italic,
                    ),
                    enabledBorder: OutlineInputBorder(
                      borderSide: BorderSide(color: Colors.green),
                      borderRadius: BorderRadius.circular(35.0),
                    ),
                    focusedBorder: OutlineInputBorder(

                      borderSide: BorderSide(color: Colors.blueAccent),
                      borderRadius: BorderRadius.circular(35.0),
                    ),
                    prefixIcon: Icon(Icons.memory),
                    filled: true,
                    fillColor: Colors.grey.shade200,
                  ),
                  textAlign: TextAlign.center,
                  textAlignVertical: TextAlignVertical.bottom,
                  onChanged: (String str) {
                    setState(() {
                      password = str;
                    });
                  },
                ),
                SizedBox(
                  height: 50.0,
                ),
                Center(
                  child: FlatButton(
                    shape: RoundedRectangleBorder(
                      borderRadius: new BorderRadius.circular(30.0),
                      side: BorderSide(color: Colors.green.shade400),
                    ),
                    color: Colors.green.shade400,
                    textColor: Colors.grey.shade300,
                    padding:
                        EdgeInsets.symmetric(horizontal: 120.0, vertical: 10.0),
                    child: Padding(
                      padding: const EdgeInsets.all(8.0),
                      child: Text(
                        'SUBMIT',
                        style: TextStyle(
                          fontSize: 18.0,
                          letterSpacing: 4.0,
                          fontStyle: FontStyle.italic,
                        ),
                      ),
                    ),
                    onPressed: () async {
                      if (_formKey.currentState.validate()) {
                        setState(() {
                          loading = true;
                        });
                       var result= "t";
                        if (result == null) {
                          setState(() {
                            loading = false;
                            error = "please supply a valid email";
                          });
                        }
                      }
                    },
                  ),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}