How to implement queries of database for flutter windows application? - database

Question

I have created windows flutter windows application now trying to save local data. I used shared_preferences plugin but it does not support windows applications so I found another package to save local data, sqflite_common_ffi plugin.
But don't know how to use it. I followed it's example and it worked but now want to separate it's all functionalities like I want to create a database in a method , and insert values in another method, get values in another method and delete them in another method.
CODE
import 'package:sqflite_common/sqlite_api.dart';
import 'package:sqflite_common_ffi/sqflite_ffi.dart';
class Storages{
var databaseFactory = databaseFactoryFfi;
Database db;
createDatabase ()async{
// Init ffi loader if needed.
sqfliteFfiInit();
db = await databaseFactory.openDatabase(inMemoryDatabasePath);
await db.execute('''
CREATE TABLE Credential(
id INTEGER PRIMARY KEY,
token TEXT,
password TEXT
)
''');
}
saveToken(String token)async{
await db.insert('Credential', <String, dynamic>{'token': token});
}
savePassword(String password)async{
await db.insert('Credential', <String, dynamic>{'password': password});
}
getToken()async{
var result = await db.query('Credential');
print('Credential token database;$result');
// await db.close();
}
getPassword()async{
var result = await db.query('Credential');
print('Credential password database;$result');
}
}
then first I initialized database saved the password in database from login screen but when I restart my application
and I called getpassword method from initState of my splash screen.
Storages storage = new Storages();
#override
void initState() {
super.initState();
storage.getPassword();
}
and thrown me this exception in my terminal
Unhandled Exception: NoSuchMethodError: The method 'query' was called on null.
Receiver: null
Tried calling: query("Credential")