GlobalKey.currentState == null

foobar.dart

import 'package:flutter/cupertino.dart';

class FooBarView extends StatefulWidget {
  VoidCallback onTap;
  String name;

  FooBarView({Key key, @required this.name, @required this.onTap}): super(key: key);

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

class FooBarViewState extends State<FooBarView> {
  int i = 0;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(onTap: widget.onTap, child: Center(child:Container(child: Text("${widget.name} - $i"))));
  }
}

home_page.dart

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  final controller = PageController();

  final fooState = GlobalKey<FooBarViewState>();
  final barState = GlobalKey<FooBarViewState>();

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: PageView(
        controller: controller,
        children: <Widget>[
          FooBarView(
            key: fooState,
            name: "Flutter",
            onTap: () {
              controller.animateToPage(1, duration: Duration(milliseconds: 300), curve: Curves.easeOut);
              barState.currentState.i++;
            },
          ),
          FooBarView(
            name: "MDV",
            key: barState,
            onTap: () {
              controller.animateToPage(0, duration: Duration(milliseconds: 300), curve: Curves.easeOut);
              fooState.currentState.i++;
            },
          ),
        ],
      ),
    );
  }
}

main.dart

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

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