Datime value showing null - flutter

Question

I have a field in my cloud firestore and there is posted_date it's showing perfectly there but when i try to put it in Text(' ${word.date_posted}') it's showing null. Help me out:
Here is an image of my database:
words_list.dart(This one calls the wordstile):
import 'package:flutter/material.dart';
import 'package:kopala_dictionary/models/words.dart';
import 'package:kopala_dictionary/screens/home/words_tile.dart';
import 'package:provider/provider.dart';
class WordsList extends StatefulWidget {
#override
_WordsListState createState() => _WordsListState();
}
class _WordsListState extends State<WordsList> {
#override
Widget build(BuildContext context) {
final words = Provider.of<List<Words>>(context) ?? [];
return ListView.builder(
itemCount: words.length,
itemBuilder: (context, index) {
return WordsTile(
word: words[index],
);
},
);
}
}
logged_home.dart(It displays all from words_tile and words_list):
import 'package:flutter/material.dart';
import 'package:kopala_dictionary/main/about.dart';
import 'package:kopala_dictionary/models/words.dart';
import 'package:kopala_dictionary/screens/home/words_list.dart';
import 'package:kopala_dictionary/screens/wrapper.dart';
import 'package:kopala_dictionary/services/auth.dart';
import 'package:kopala_dictionary/services/database.dart';
import 'package:kopala_dictionary/shared/app_bar.dart';
import 'package:provider/provider.dart';
class LoggedInUserHome extends StatelessWidget {
final AuthService _auth = AuthService();
#override
Widget build(BuildContext context) {
return StreamProvider<List<Words>>.value(
value: DatabaseService().words,
child: Scaffold(
backgroundColor: Colors.green[10],
appBar: LoggedBar(),
drawer: Drawer(
child: ListView(
padding: EdgeInsets.zero,
children: <Widget>[
DrawerHeader(
child: Text(
'Kopalationary Menu',
style: TextStyle(color: Colors.white),
),
decoration: BoxDecoration(
color: Colors.green[800],
),
),
ListTile(
title: Text('Home'),
onTap: () {
Navigator.push(context,
MaterialPageRoute(builder: (context) => Wrapper()));
},
),
ListTile(
title: Text(
'My profile',
),
onTap: () {},
),
ListTile(
title: Text('Logout'),
onTap: () async {
dynamic result = await _auth.logoutUser();
},
),
ListTile(
title: Text(
'About',
),
onTap: () {
Navigator.push(
context,
MaterialPageRoute(builder: (context) => About()),
);
},
),
],
),
),
body: WordsList(),
),
);
}
}
words_tile.dart:
import 'package:flutter/material.dart';
import 'package:kopala_dictionary/models/words.dart';
class WordsTile extends StatelessWidget {
final Words word;
WordsTile({this.word});
#override
Widget build(BuildContext context) {
return Padding(
padding: EdgeInsets.only(top: 8.0),
child: Card(
margin: EdgeInsets.fromLTRB(5.0, 8.0, 5.0, 0.0),
child: ListTile(
leading: CircleAvatar(
backgroundColor: Colors.green[800],
radius: 25.0,
),
title: Text(word.word),
subtitle: Text(' ${word.date_posted}'),
),
),
);
}
}
It's showing null: