NoSuchMethodError was thrown building Tabs(dirty, state: _TabsState#fefcf): The method '[]' was called on null - flutter

Question

I have read similar answers for this type of error but still, I am not able to resolve it in my code.
When I tap the button it shows an error screen for some time and then displays loader which should be displayed while the data is being fetched.
error screen
loading screen
final screen
here is the code:
import 'package:flutter/material.dart';
import 'Dictionary.dart';
import 'Services.dart';
class NewTab extends StatefulWidget {
#override
_NewTabState createState() => _NewTabState();
}
class _NewTabState extends State<NewTab> {
List<Dictionary> _users;
int len;
#override
void initState() {
super.initState();
Services.getUsers().then((users) {
setState(() {
_users = users;
len = users.length - 1 ;
});
});
}
#override
Widget build(BuildContext context) {
Dictionary user = _users[len];
print(user);
return FutureBuilder(
future: Services.getUsers(),
builder: (context,AsyncSnapshot<List<Dictionary>> snapshot){
if(snapshot.data==null){
return Center(child: CircularProgressIndicator());
}else{
return DefaultTabController(length: 2,
child:Scaffold(
appBar: AppBar(
title : Text('Meaning',
style: TextStyle(
fontFamily: "Pangolin",
fontSize: 35,
)
),
centerTitle: true,
bottom: TabBar(
tabs: <Widget>[
Tab(text:'Meaning',
icon: Icon(Icons.book_outlined),
),
Tab(text:'Example',
icon: Icon(Icons.directions_transit)
),
],
),
),
body : TabBarView(
children: <Widget>[
Container(
color: Colors.white,
child: Container(
color: Colors.black,
child: ListView.builder(
itemCount: user.meanings.length,
itemBuilder: (context, index) {
print(index);
List<Meaning> mean = user.meanings;
return ListTile(
title: Text(mean[index].definitions[0].definition
, style: TextStyle(
color: Colors.white,
),
),
);
},
),
),),Container(
child: Text('home'),
),]
)
)
);
}
},
);
}
}
code for onpressed function in raised button
onPressed: ()=>{
Navigator.of(context).push(
new MaterialPageRoute(builder: (BuildContext context) => NewTab())
)
},
code for services.dart
import 'package:http/http.dart' as http;
import 'Dictionary.dart';
import 'package:dictionary_app/home.dart';
import 'dart:async';
class Services {
static final url = 'https://api.dictionaryapi.dev/api/v2/entries/en_US/$word';
static Future<List<Dictionary>> getUsers() async {
try {
print(url);
final response = await http.get(Uri.parse(url));
print(response.statusCode);
if (200 == response.statusCode) {
final List<Dictionary> users = dictionaryFromJson(response.body);
return users;
}
else {
return <Dictionary>[];
}
} catch (e) {
return <Dictionary>[];
}
}
}
error:
The following NoSuchMethodError was thrown building NewTab(dirty, state: _NewTabState#1662b):
The method '[]' was called on null.
Receiver: null
Tried calling: [](null)
The relevant error-causing widget was:
NewTab file:///E:/Flutter-Projects/flutter3/dictionary_app/lib/home.dart:120:74
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:63:5)
#1 _NewTabState.build (package:dictionary_app/newtab.dart:25:30)
#2 StatefulElement.build (package:flutter/src/widgets/framework.dart:4749:27)
#3 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4632:15)
#4 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4805:11)
#5 Element.rebuild (package:flutter/src/widgets/framework.dart:4322:5)
#6 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4610:5)
#7 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4796:11)
#8 ComponentElement.mount (package:flutter/src/widgets/framework.dart:4605:5)
... Normal element mounting (166 frames)
#174 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3662:14)
#175 MultiChildRenderObjectElement.inflateWidget (package:flutter/src/widgets/framework.dart:6285:36)
#176 Element.updateChild (package:flutter/src/widgets/framework.dart:3414:18)
#177 RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:5717:32)
#178 MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6308:17)
#179 Element.updateChild (package:flutter/src/widgets/framework.dart:3401:15)
#180 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4657:16)
#181 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4805:11)
#182 Element.rebuild (package:flutter/src/widgets/framework.dart:4322:5)
#183 StatefulElement.update (package:flutter/src/widgets/framework.dart:4837:5)
#184 Element.updateChild (package:flutter/src/widgets/framework.dart:3401:15)
#185 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4657:16)
#186 Element.rebuild (package:flutter/src/widgets/framework.dart:4322:5)
#187 ProxyElement.update (package:flutter/src/widgets/framework.dart:4985:5)
#188 Element.updateChild (package:flutter/src/widgets/framework.dart:3401:15)
#189 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4657:16)
#190 Element.rebuild (package:flutter/src/widgets/framework.dart:4322:5)
#191 ProxyElement.update (package:flutter/src/widgets/framework.dart:4985:5)
#192 _InheritedNotifierElement.update (package:flutter/src/widgets/inherited_notifier.dart:181:11)
#193 Element.updateChild (package:flutter/src/widgets/framework.dart:3401:15)
#194 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6154:14)
#195 Element.updateChild (package:flutter/src/widgets/framework.dart:3401:15)
#196 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4657:16)
#197 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4805:11)
#198 Element.rebuild (package:flutter/src/widgets/framework.dart:4322:5)
#199 StatefulElement.update (package:flutter/src/widgets/framework.dart:4837:5)
#200 Element.updateChild (package:flutter/src/widgets/framework.dart:3401:15)
#201 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6154:14)
#202 Element.updateChild (package:flutter/src/widgets/framework.dart:3401:15)
#203 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6154:14)
#204 Element.updateChild (package:flutter/src/widgets/framework.dart:3401:15)
#205 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4657:16)
#206 Element.rebuild (package:flutter/src/widgets/framework.dart:4322:5)
#207 ProxyElement.update (package:flutter/src/widgets/framework.dart:4985:5)
#208 Element.updateChild (package:flutter/src/widgets/framework.dart:3401:15)
#209 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4657:16)
#210 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4805:11)
#211 Element.rebuild (package:flutter/src/widgets/framework.dart:4322:5)
#212 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2619:33)
#213 WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:878:21)
#214 RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:328:5)
#215 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1147:15)
#216 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1084:9)
#217 SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:1000:5)
#221 _invoke (dart:ui/hooks.dart:163:10)
#222 PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:259:5)
#223 _drawFrame (dart:ui/hooks.dart:126:31)
(elided 3 frames from dart:async)

Answer 1

First of all you can remove your initState method since you are already using the FutureBuilder to fetch your Services.getUsers().
Now, you have defined your variables like this,
List<Dictionary> _users;
int len;
But both of them are null when build method is going to be called for the first time.
So inside your build method, you can't directly use,
Dictionary user = _users[len];
print(user);
Since, _users is null when build is called for the first time. So remove those 2 lines.
Now, rest of your code is correct, except for one part,
Inside your else case of if(snapshot.data==null), define your user variable like this,
else {
Dictionary user = snapshot.data!.last; // This gives you the last element of the array
Now, your code should work.
Here is the paste bin link for the full code of _NewTabState.

Answer 2

You have to initialize your list
List<Dictionary> _users = [];
It’s better way:
snapShot.connectionState == ConnectionState.waiting ? :