[Flutter] Flutter: Unhandled Exception: Null check operator used on a null value

I am practicing with SQFLite database in flutter App. I want to create a database where user can add friend's name and mobile number.



And after adding this information, that information list will be shown under submit button [ where I using listview.builder() ].

Everything is fine. Data is adding to database. (I checked .db file by manually also). But when I refresh the list it's show me a error like this -

flutter: Done Fetch
[VERBOSE-2:ui_dart_state.cc(199)] Unhandled Exception: Null check operator used on a null value
#0 QueryResultSet.columnIndex (package:sqflite_common/src/collection_utils.dart:114:32)
#1 QueryRow.[] (package:sqflite_common/src/collection_utils.dart:135:40)
#2 new Contact.fromMap (package:sample/model/contact.dart:25:15)
#3 DatabaseHelper.fetchContacts.<anonymous closure> (package:sample/utils/database_helper.dart:63:39)
#4 MappedListIterable.elementAt (dart:_internal/iterable.dart:412:31)
#5 ListIterator.moveNext (dart:_internal/iterable.dart:341:26)
#6 new _GrowableList._ofEfficientLengthIterable (dart:core-patch/growable_array.dart:188:27)
#7 new _GrowableList.of (dart:core-patch/growable_array.dart:150:28)
#8 new List.of (dart:core-patch/array_patch.dart:50:28)
#9 ListIterable.toList (dart:_internal/iterable.dart:212:44)
#10 DatabaseHelper.fetchContacts (package:sample/utils/database_helper.dart:63:51)
<asynchronous suspension>


Data fetch function also run perfectly. (Console show me the print("Done Fetch");'s output). I think there is a problem in refresh function.

Please someone help me. Where is the problem , and how Can I solve it ?

UI Screen =>
​

import 'package:flutter/material.dart';
.....

class FlutterLocalDatabase extends StatefulWidget {
@override
_FlutterLocalDatabaseState createState() => _FlutterLocalDatabaseState();
}

class _FlutterLocalDatabaseState extends State<FlutterLocalDatabase> {
....
Contact _contact = Contact();
List<Contact> contactList = [];

DatabaseHelper _dbHelper;
@override
void initState() {
super.initState();
setState(() {
_dbHelper = DatabaseHelper.instance;
});

refreshContactList();
}

@override
Widget build(BuildContext context) {
double _width = MediaQuery.of(context).size.width;
double _height = MediaQuery.of(context).size.height;
return Scaffold(
appBar: AppBar(
title: Text("Flutter SQFLite Database"),
),
body: GestureDetector(
......
.....
ElevatedButton(
onPressed: () async {
print("${_contact.name}, ${_contact.mobile}");
if (validateAndSave(_formKey)) {
FocusScope.of(context).requestFocus(new FocusNode());
if (_contact.id == null) {
await _dbHelper.insertContact(_contact).then((value) {
print(value);
_resetForm();
refreshContactList();
});
}

print(contactList);
}
},
child: Text("Submit")),
SizedBox(
height: 20,
),
Expanded(
child: Container(
child: ListView.builder(
itemCount: contactList.length,
itemBuilder: (buildContext, index) {
var contact = contactList[index];
return Card(
child: ListTile(
leading: Icon(
Icons.account_circle,
size: 40,
color: Colors.teal,
),
title: Text(
contact.name,
style: TextStyle(
color: Colors.teal,
),
),
subtitle: Text(contact.mobile,
style: TextStyle(
color: Colors.teal,
)),
trailing: Text("${contact.id}",
style: TextStyle(
color: Colors.teal,
)),
),
);
}),
))
],
),
),
),
),
);
}

refreshContactList() async {
List<Contact> x = await _dbHelper.fetchContacts();
print("Done Refresh");
setState(() {
contactList = x;
});
}

_resetForm() {
setState(() {
_formKey.currentState.reset();
_ctrlName.clear();
_ctrlMobile.clear();
_contact.id = null;
});
}
}

Model Class =>
​

class Contact {
static const tblContact = "contacts";
static const colId = "id";
static const colName = "name";
static const colMobile = "mobile";

Contact({
this.id,
this.name,
this.mobile,
});

int id;
String name;
String mobile;

Map<String, dynamic> toMap() {
Map map = <String, dynamic>{colName: name, colMobile: mobile};
if (id != null) map[colId] = id;
return map;
}

Contact.fromMap(Map<String, dynamic> map) {
id = map[colId];
name = map[name];
mobile = map[colMobile];
}
}

Database Helper Class =>
​

import 'dart:io';
.....
class DatabaseHelper {
static const _databaseName = "ContactData.db";
static const _databaseVersion = 1;

//<====== Singleton Class
DatabaseHelper._();
static final DatabaseHelper instance = DatabaseHelper._();

Database _database;
Future<Database> get database async {
if (_database != null) {
return _database;
} else {
_database = await _initDatabase();
return _database;
}
}

//CREATE DATABASE
_initDatabase() async {
Directory dataDirectory = await getApplicationDocumentsDirectory();
String dbPath = join(dataDirectory.path, _databaseName);
print(dbPath);
return await openDatabase(dbPath,
version: _databaseVersion, onCreate: _onCreate);
}

//CREATE TABLE
_onCreate(Database db, int version) async {
db.execute('''
CREATE TABLE ${Contact.tblContact}(
${Contact.colId} INTEGER PRIMARY KEY AUTOINCREMENT,
${Contact.colName} STRING NOT NULL,
${Contact.colMobile} STRING NOT NULL
);
''');
print("Done on Create");
}

//<=================== ADD DATA

Future<int> insertContact(Contact contact) async {
Database db = await database;
print("Done on Insert");
return await db.insert(Contact.tblContact, contact.toMap());
}

//<==================== Read Data
Future<List<Contact>> fetchContacts() async {
Database db = await database;
List<Map> contacts = await db.query(Contact.tblContact);
print("Done Fetch");
return contacts.length == 0
? []
: contacts.map((x) => Contact.fromMap(x)).toList();
}

}

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