Flutter app exit error

main.dart

import 'dart:io';

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

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or press Run > Flutter Hot Reload in IntelliJ). Notice that the
        // counter didn't reset back to zero; the application is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return new Scaffold(
      appBar: new AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: new Text(widget.title),
      ),
      body: new CupertinoTabScaffold(
        tabBar: new CupertinoTabBar(
          items: <BottomNavigationBarItem> [
            new BottomNavigationBarItem(
                backgroundColor: Colors.white,
                icon: new Icon(Icons.tab),
                title: new Text("tab1")
            ),
            new BottomNavigationBarItem(
                backgroundColor: Colors.white,
                icon: new Icon(Icons.tab),
                title: new Text("tab2")
            ),new BottomNavigationBarItem(
                backgroundColor: Colors.white,
                icon: new Icon(Icons.tab),
                title: new Text("tab3")
            ),


          ],
        ),
        tabBuilder: (BuildContext context, int index) {
          if (index == 2){
            ()=>exit(0);
          }else{
          return new CupertinoTabView(
            builder: (BuildContext context) {
              return new CupertinoPageScaffold(
                navigationBar: new CupertinoNavigationBar(
                  middle: new Text('Page 1 of tab $index'),
                ),
                child: new Center(
                  child: new CupertinoButton(
                    child: const Text('Next page'),
                    onPressed: () {
                      Navigator.of(context).push(
                        new CupertinoPageRoute<Null>(
                          builder: (BuildContext context) {
                            return new CupertinoPageScaffold(
                              navigationBar: new CupertinoNavigationBar(
                                middle: new Text('Page 2 of tab $index'),
                              ),
                              child: new Center(
                                child: new CupertinoButton(
                                  child: const Text('Back'),
                                  onPressed: () { Navigator.of(context).pop(); },
                                ),
                              ),
                            );
                          },
                        ),
                      );
                    },
                  ),
                ),
              );
            },
          );
        }
        },
      )
    );
  }
}