Non-nullable instance field ['controller'] must be initialized in flutter

Since your field does not allow null values, but you cannot initialize your field when it's created, you have to declare it as late:

late TabController controller;

This allows you to assign a value later, but you will get an exception if you use it before it was assigned. The way your code looks, it should be okay.

Plase try, you missed use controller

import 'package:flutter/material.dart';
import 'sub/firstPage.dart';
import 'sub/secondPage.dart';


void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(

        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key? key, required this.title}) : super(key: key);



  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> with SingleTickerProviderStateMixin {

  late TabController controller;

  @override
  void initState() {
    super.initState();
    controller = TabController(length: 2, vsync: this);


  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text('TabBar Example'),
        ),
        body: TabBarView(
          children: <Widget>[FirstApp(), SecondApp()],
          controller: controller,
        ),
        bottomNavigationBar: TabBar(
          controller: controller, // add here 
          tabs: [
          Tab(icon: Icon(Icons.looks_one, color: Colors.blue),),
          Tab(icon: Icon(Icons.looks_two, color: Colors.blue),),
        ],
        )
    );
  }

  @override
  void dispose() {
    controller.dispose();
    super.dispose();
  }


}

It looks like you are using null safety in your project. So the dart is complaining about this line. Dart expects it to be initialized right after it was created:

TabController controller;

To fix the problem add the "late" keyword before TabController:

late TabController controller;