Failed assertion issue on login page state

The following assertion was thrown building LoginPage(dirty, state: _LoginPageState#bf00d): A non-null String must be provided to a Text widget. 'package:flutter/src/widgets/text.dart': Failed assertion: line 360 pos 10: 'data != null'

I have this error and looks like no issue in text.dart.

This is my loginPage.dart page.

I can't find the problem.

import 'package:flutter/material.dart';
import 'package:sporprojesi/login/primary_button.dart';
import 'package:sporprojesi/login/signin.dart';



class LoginPage extends StatefulWidget {
  LoginPage({Key key, this.title, this.auth, this.onSignIn}) : super(key: key);

  final String title;
  final BaseAuth auth;
  final VoidCallback onSignIn;

  @override
  _LoginPageState createState() => new _LoginPageState();
}

enum FormType {
  login,
  register
}

class _LoginPageState extends State<LoginPage> {
  static final formKey = new GlobalKey<FormState>();

  String _email;
  String _password;
  FormType _formType = FormType.login;
  String _authHint = '';


  @override
  void initState() {

    super.initState();}

  bool validateAndSave() {
    final form = formKey.currentState;
    if (form.validate()) {
      form.save();
      return true;
    }
    return false;
  }

  void validateAndSubmit() async {
    if (validateAndSave()) {
      try {
        String userId = _formType == FormType.login
            ? await widget.auth.signIn(_email, _password)
            : await widget.auth.createUser(_email, _password);
        setState(() {
          _authHint = 'Signed In\n\nUser id: $userId';
        });
        widget.onSignIn();
      }
      catch (e) {
        setState(() {
          _authHint = 'Sign In Error\n\n${e.toString()}';
        });
        print(e);
      }
    } else {
      setState(() {
        _authHint = '';
      });
    }
  }

  void moveToRegister() {
    formKey.currentState.reset();
    setState(() {
      _formType = FormType.register;
      _authHint = '';
    });
  }

  void moveToLogin() {
    formKey.currentState.reset();
    setState(() {
      _formType = FormType.login;
      _authHint = '';
    });
  }

  List<Widget> usernameAndPassword() {
    return [
      padded(child: new TextFormField(
        key: new Key('email'),
        decoration: new InputDecoration(labelText: 'Email'),
        autocorrect: false,
        validator: (val) => val.isEmpty ? 'Email can\'t be empty.' : "",
        onSaved: (val) => _email = val,
      )),
      padded(child: new TextFormField(
        key: new Key('password'),
        decoration: new InputDecoration(labelText: 'Password'),
        obscureText: true,
        autocorrect: false,
        validator: (val) => val.isEmpty ? 'Password can\'t be empty.' : "",
        onSaved: (val) => _password = val,
      )),
    ];
  }

  List<Widget> submitWidgets() {
    switch (_formType) {
      case FormType.login:
        return [
          new PrimaryButton(
              key: new Key('login'),
              text: 'Login',
              height: 44.0,
              onPressed: validateAndSubmit
          ),
          new FlatButton(
              key: new Key('need-account'),
              child: new Text("Need an account? Register"),
              onPressed: moveToRegister
          ),
        ];
      case FormType.register:
        return [
          new PrimaryButton(
              key: new Key('register'),
              text: 'Create an account',
              height: 44.0,
              onPressed: validateAndSubmit
          ),
          new FlatButton(
              key: new Key('need-login'),
              child: new Text("Have an account? Login"),
              onPressed: moveToLogin
          ),
        ];
    }
    return null;
  }

  Widget hintText() {
    return new Container(
      //height: 80.0,
        padding: const EdgeInsets.all(32.0),
        child: new Text(
            _authHint,
            key: new Key('hint'),
            style: new TextStyle(fontSize: 18.0, color: Colors.grey),
            textAlign: TextAlign.center)
    );
  }


  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        appBar: new AppBar(
          title: new Text(widget.title),
        ),
        backgroundColor: Colors.grey[300],
        body: new SingleChildScrollView(child: new Container(
            padding: const EdgeInsets.all(16.0),
            child: new Column(
                children: [
                  new Card(
                      child: new Column(
                          mainAxisSize: MainAxisSize.min,
                          children: <Widget>[
                            new Container(
                                padding: const EdgeInsets.all(16.0),
                                child: new Form(
                                    key: formKey,
                                    child: new Column(
                                      crossAxisAlignment: CrossAxisAlignment.stretch,
                                      children: usernameAndPassword() + submitWidgets(),
                                    )
                                )
                            ),
                          ])
                  ),
                  hintText()
                ]
            )
        ))
    );
  }

  Widget padded({Widget child}) {
    return new Padding(
      padding: EdgeInsets.symmetric(vertical: 8.0),
      child: child,
    );
  }
}