Error: NoSuchMethodError: 'then' Dynamic call of null. Receiver: null - flutter

Question

Does anyone know the cause of this error? I have tried many ways but still don't know where the problem is.
Database.dart
import 'package:fitness_app/Login/login_data.dart';
import 'package:path/path.dart';
import 'package:sqflite/sqflite.dart';
class DatabaseService {
static final DatabaseService _databaseService = DatabaseService._internal();
factory DatabaseService() => _databaseService;
DatabaseService._internal();
static Database? _database;
Future<Database> get database async {
if (_database != null) return _database!;
_database = await _initDatabase();
return _database!;
}
Future<Database> _initDatabase() async {
final databasePath = await getDatabasesPath();
final path = join(databasePath, 'conference.database');
return await openDatabase(
path,
onCreate: _onCreate,
version: 1,
onConfigure: (db) async => await db.execute('PRAGMA foreign_keys = ON'),
);
}
Future<void> _onCreate(Database db, int version) async {
await db.execute(
'CREATE TABLE login(id INTEGER PRIMARY KEY, name TEXT, username TEXT, password TEXT)',
);
}
verifyuser(String user, String pass) {}
insertLoginData(Logindata logindata) {}
}
Login.dart
import 'package:fitness_app/Login/signup.dart';
import 'package:flutter/material.dart';
import 'login_data.dart';
import 'package:fitness_app/Database/database.dart';
import 'package:fitness_app/home_page.dart';
class Login extends StatefulWidget {
const Login({Key? key, this.login}) : super(key: key);
final Logindata? login;
#override
_LoginState createState() => _LoginState();
}
class _LoginState extends State<Login> {
String email = "a";
String pass = "a";
TextEditingController emails = TextEditingController();
TextEditingController password = TextEditingController();
final _formKey = GlobalKey<FormState>();
static final List<Logindata> _login = [];
final DatabaseService _databaseService = DatabaseService();
Future<List<Logindata>> _getLogin() async {
await _databaseService.verifyuser(email, pass).then((value) {
if (value) {
AlertDialog alert = AlertDialog(
title: const Text('Login successful!'),
content: const Text('Welcome!'),
actions: <Widget>[
TextButton(
onPressed: () => (Navigator.push(
context,
MaterialPageRoute(builder: (context) => const HomePage()),
)),
child: const Text('OK'),
),
],
);
showDialog(
context: context,
builder: (BuildContext context) {
return alert;
},
);
} else {
AlertDialog alert = AlertDialog(
title: const Text('Error!'),
content: const Text('Wrong Email or Password'),
actions: <Widget>[
TextButton(
onPressed: () => Navigator.pop(context, 'OK'),
child: const Text('OK'),
),
],
);
showDialog(
context: context,
builder: (BuildContext context) {
return alert;
},
);
}
});
return _login;
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: SingleChildScrollView(
child: Column(
children: [
Container(
padding: EdgeInsets.all(10),
child: Form(
key: _formKey,
child: Column(
children: [
Text(
'BeFit:Fitness Activity Tracker Progress\n\n',
style: TextStyle(fontSize: 24),
textAlign: TextAlign.start,
),
Text(
'Welcome',
style: TextStyle(fontSize: 40),
textAlign: TextAlign.center,
),
SizedBox(
height: 30,
),
Container(
child: TextFormField(
controller: emails,
decoration: InputDecoration(
border: UnderlineInputBorder(
borderSide:
BorderSide(width: 1, color: Colors.grey)),
labelText: 'Email'),
validator: (value) {
if (value == null || value.isEmpty) {
return 'Email is required';
}
return null;
},
),
),
SizedBox(
height: 20,
),
Container(
child: TextFormField(
controller: password,
obscureText: true,
decoration: InputDecoration(
border: UnderlineInputBorder(
borderSide:
BorderSide(width: 1, color: Colors.grey)),
labelText: 'Password'),
validator: (value) {
if (value == null || value.isEmpty) {
return 'Password is required';
}
return null;
},
),
),
SizedBox(
height: 20,
),
Container(
width: MediaQuery.of(context).size.width,
height: 50,
child: FlatButton(
onPressed: () {
if (_formKey.currentState!.validate()) {
email = emails.text;
pass = password.text;
_getLogin();
print(email);
print(pass);
print('success');
}
},
child: Text("Login"),
textColor: Colors.white,
color: Colors.deepPurple[400],
shape: new RoundedRectangleBorder(
borderRadius: new BorderRadius.circular(10)),
),
),
SizedBox(
height: 20,
),
Container(
child: Row(
children: <Widget>[
Text('Does not have account?'),
FlatButton(
textColor: Colors.deepPurpleAccent[100],
child: Text(
'Sign up',
style: TextStyle(fontSize: 16),
),
onPressed: () {
//signup screen
Navigator.push(
context,
MaterialPageRoute(
builder: (context) => const SignUp()),
);
},
)
],
mainAxisAlignment: MainAxisAlignment.center,
))
],
),
),
)
],
),
),
),
);
}
}
Here is the error
enter image description here
I'm using this flutter to complete my project but there are some errors that I can't solve it. Sorry if my coding looks not right because still lacks in coding

Answer 1

Your verifyuser function does not return anything so your value returns null. You either need to return something or check for null values in your then statement.
Future<String> verifyuser(String user, String pass)async {
return user + pass;
}