alertDialog with textfield validator flutter

  // Creates an alertDialog for the user to enter their email
  Future<String> _resetDialogBox() {

    return showDialog<String>(
      context: context,
      barrierDismissible: false, // user must tap button!
      builder: (BuildContext context) {
        return CustomAlertDialog(
          title: "Reset email",
          auth: _auth,
        );
      },
    );
  }


class CustomAlertDialog extends StatefulWidget {
  final String title;
  final FirebaseAuth auth;

  const CustomAlertDialog({Key key, this.title, this.auth})
      : super(key: key);

  @override
  CustomAlertDialogState createState() {
    return new CustomAlertDialogState();
  }
}

class CustomAlertDialogState extends State<CustomAlertDialog> {

  final _resetKey = GlobalKey<FormState>();
  final _resetEmailController = TextEditingController();
  String _resetEmail;
  bool _resetValidate = false;

  StreamController<bool> rebuild = StreamController<bool>();

  bool _sendResetEmail() {
    _resetEmail = _resetEmailController.text;

    if (_resetKey.currentState.validate()) {
      _resetKey.currentState.save();

      try {
        // You could consider using async/await here
        widget.auth.sendPasswordResetEmail(email: _resetEmail);
        return true;
      } catch (exception) {
        print(exception);
      }
    } else {
      setState(() {
        _resetValidate = true;
      });
      return false;
    }
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: AlertDialog(
        title: new Text(widget.title),
        content: new SingleChildScrollView(
            child: Form(
              key: _resetKey,
              autovalidate: _resetValidate,
              child: ListBody(
                children: <Widget>[
                  new Text(
                    'Enter the Email Address associated with your account.',
                    style: TextStyle(fontSize: 14.0),
                  ),
                  Padding(
                    padding: EdgeInsets.all(10.0),
                  ),
                  Row(
                    children: <Widget>[
                      new Padding(
                        padding: EdgeInsets.only(top: 8.0),
                        child: Icon(
                          Icons.email,
                          size: 20.0,
                        ),
                      ),
                      new Expanded(
                        child: TextFormField(
                          validator: validateEmail,
                          onSaved: (String val) {
                            _resetEmail = val;
                          },
                          controller: _resetEmailController,
                          keyboardType: TextInputType.emailAddress,
                          autofocus: true,
                          decoration: new InputDecoration(
                              border: InputBorder.none,
                              hintText: 'Email',
                              contentPadding:
                              EdgeInsets.only(left: 70.0, top: 15.0),
                              hintStyle:
                              TextStyle(color: Colors.black, fontSize: 14.0)),
                          style: TextStyle(color: Colors.black),

                        ),
                      )
                    ],
                  ),
                  new Column(children: <Widget>[
                    Container(
                      decoration: new BoxDecoration(
                          border: new Border(
                              bottom: new BorderSide(
                                  width: 0.5, color: Colors.black))),
                    )
                  ]),
                ],
              ),
            ),
        ),
        actions: <Widget>[
          new FlatButton(
            child: new Text(
              'CANCEL',
              style: TextStyle(color: Colors.black),
            ),
            onPressed: () {
              Navigator.of(context).pop("");
            },
          ),
          new FlatButton(
            child: new Text(
              'SEND EMAIL',
              style: TextStyle(color: Colors.black),
            ),
            onPressed: () {
              if (_sendResetEmail()) {
                Navigator.of(context).pop(_resetEmail);
              }
            },
          ),
        ],
      ),
    );
  }
}

String validateEmail(String value) {
  String pattern =
      r'^(([^<>()[\]\\.,;:\s@\"]+(\.[^<>()[\]\\.,;:\s@\"]+)*)|(\".+\"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$';
  RegExp regExp = new RegExp(pattern);
  if (value.length == 0) {
    return "Email is required";
  } else if (!regExp.hasMatch(value)) {
    return "Invalid Email";
  } else {
    return null;
  }
}