Heroes Failed assertion: line 248 pos 12: 'box != null && box.hasSize': is not true.

main.dart

import 'package:flutter/material.dart';

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

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

class PageOne extends StatelessWidget {
  void navigate(BuildContext context) {
    Navigator.of(context).push(new MaterialPageRoute(
        builder: (BuildContext context) => new PageTwo()));
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new GestureDetector(
        onTap: () => navigate(context),
        child: new Box(
          color: Colors.green,
          height: 100,
          width: 100,
        ),
      ),
    );
  }
}

class PageTwo extends StatelessWidget {
  void navigate(BuildContext context) {
    Navigator.of(context).push(new MaterialPageRoute(
        builder: (BuildContext context) => new PageThree()));
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new GestureDetector(
        onTap: () => navigate(context),
        child: new Box(
          color: Colors.orange,
          height: 200,
          width: 80,
        ),
      ),
    );
  }
}

class PageThree extends StatelessWidget {
  void navigate(BuildContext context) {
    Navigator.of(context).pushAndRemoveUntil(
      new MaterialPageRoute(builder: (BuildContext context) => new PageOne()),
      (Route<dynamic> route) => route.isFirst,
    );
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new GestureDetector(
        onTap: () => navigate(context),
        child: new Box(
          color: Colors.blue,
          height: 150,
          width: 200,
        ),
      ),
    );
  }
}

class Box extends StatelessWidget {
  final Color color;
  final double height;
  final double width;

  const Box({Key key, this.color, this.height, this.width}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return new Center(
      child: new Hero(
        tag: "hero",
        child: new Container(
          color: color,
          margin: EdgeInsets.all(80),
          height: height,
          width: width,
        ),
      ),
    );
  }
}