Unhandled Exception: LateInitializationError: Field '_db#19320762' has not been initialized - flutter

Question

import 'package:flutter/material.dart';
import 'package:sqlflite_demo/screens/product_list.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
// TODO: implement build
return MaterialApp(
home: ProductList(),
);
}
}
main.dart
class Product {
int? id;
String? name;
String? description;
double? unitPrice;
Product({this.name, this.description, this.unitPrice});
Product.withId({this.id, this.name, this.description, this.unitPrice});
Map<String,dynamic> toMap(){
var map = Map<String,dynamic>();
map["name"]=name;
map["description"]=description;
map["unitPrice"]=unitPrice;
if(id!=null){
map["id"]=id;
}
return map;
}
Product.fromObject(dynamic o){
this.id = int.tryParse(o["id"]);
this.name=o["name"];
this.description=o["description"];
this.unitPrice = double.tryParse(o["unitPrice"].toString());
}
}
product.dart
import 'dart:async';
import 'package:path/path.dart';
import 'package:sqflite/sqflite.dart';
import '../models/product.dart';
class DbHelper {
late Database _db;
Future<Database> get db async {
if (_db == null) {
_db = await initializeDb();
}
return _db;
}
Future<Database> initializeDb() async {
String dbPath = join(await getDatabasesPath(), "etrade.db");
var eTradeDb = await openDatabase(dbPath,version: 1, onCreate: createDb);
return eTradeDb;
}
void createDb(Database db, int version)async {
await db.execute("Create table products(id integer primary key, name text, description text, unitPrice integer)");
}
Future<List<Product>> getProducts() async{
Database db = await this.db;
var result = await db.query("products");
return List.generate(result.length, (i){
return Product.fromObject(result[i]);
});
}
Future<int> insert(Product product)async{
Database db = await this.db;
var result = await db.insert("products", product.toMap());
return result; ////////
}
Future<int> delete(int id)async{
Database db = await this.db;
var result = await db.rawDelete("delete from products where id=$id");
return result;
}
Future<int> update(Product product)async{
Database db = await this.db;
var result = await db.update("products", product.toMap(), where: "id=?",whereArgs: [product.id]);
return result;
}
}
db_Helper.dart
import 'package:flutter/material.dart';
import 'package:sqlflite_demo/data/dbHelper.dart';
import 'package:sqlflite_demo/models/product.dart';
import 'package:sqlflite_demo/screens/product_add.dart';
class ProductList extends StatefulWidget{
#override
State<StatefulWidget> createState() {
return _ProductListState();
}
}
class _ProductListState extends State {
var dbHelper = DbHelper();
late List<Product> products;
int productCount =0;
#override
void initState() {
getProducts();
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Ürün Listesi"),
),
body: buildProductList(),
floatingActionButton: FloatingActionButton(
onPressed: (){goToProductAdd();},
child: Icon(Icons.add),
tooltip: "Yeni Ürün Ekle",
),
);
}
ListView buildProductList() {
return ListView.builder(
itemCount: productCount,
itemBuilder: (BuildContext context, int position){
return Card(
color: Colors.pinkAccent,
elevation: 2.0,
child: ListTile(
leading: CircleAvatar(backgroundColor: Colors.black12, child: Text("P"),),
title: Text(products[position].name.toString()),
subtitle: Text(products[position].description.toString()),
onTap: (){},
),
);
});
}
void goToProductAdd() async{
bool result = await Navigator.push(context, MaterialPageRoute(builder: (context)=>ProductAdd()));
if(result!=null){
if(result){
getProducts();
}
}
}
void getProducts() async{
var productsFuture = dbHelper.getProducts();
productsFuture.then((data){
this.products = data;
productCount=data.length;
});
}
}
product_list.dart
import 'package:flutter/material.dart';
import 'package:sqlflite_demo/data/dbHelper.dart';
import '../models/product.dart';
class ProductAdd extends StatefulWidget{
#override
State<StatefulWidget> createState() {
return ProductAddState();
}
}
class ProductAddState extends State {
var dbHelper = DbHelper();
var txtName=TextEditingController();
var txtDescription=TextEditingController();
var txtUnitPrice=TextEditingController();
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Yeni Ürün Ekle"),
),
body: Padding(
padding: EdgeInsets.all(30.0),
child: Column(
children: [
buildNameField(),buildDescriptionField(),buildUnitPriceField(),buildSaveButton()
],
),
),
);
}
buildNameField() {
return TextField(
decoration: InputDecoration(labelText: "Ürün Adı"),
controller: txtName,
);
}
buildDescriptionField() {
return TextField(
decoration: InputDecoration(labelText: "Ürün Açıklaması"),
controller: txtDescription,
);
}
buildUnitPriceField() {
return TextField(
decoration: InputDecoration(labelText: "Birim Fiyatı"),
controller: txtUnitPrice,
);
}
buildSaveButton() {
return FlatButton(
child: Text("Ekle"),
onPressed: (){
addProduct();
},
);
}
void addProduct() async{
var result = await dbHelper.insert(Product(name: txtName.text, description: txtDescription.text, unitPrice: double.tryParse(txtUnitPrice.text)));
Navigator.pop(context,true);
}
}
product_add.dart
[ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: LateInitializationError: Field '_db#19320762' has not been initialized.
This is my first project in flutter. and I have an error. can you explain that? Thanks. I dont know what happened