Flutter countdown stateful widget

main.dart

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

void main() {
  runApp(MaterialApp(title: 'Countdown Timer', home: FirstRoute()));
}

class FirstRoute extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("First Route"),
        ),
        body: Center(
            child: ElevatedButton(
                child: Text('Countdown'),
                onPressed: () {
                  Navigator.push(
                    context,
                    MaterialPageRoute(builder: (context) => SecondRoute()),
                  );
                })));
  }
}

class SecondRoute extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("Second Route"),
        ),
        body: Center(
            child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                crossAxisAlignment: CrossAxisAlignment.center,
                children: [
              ElevatedButton(
                onPressed: () {
                  Navigator.pop(context);
                },
                child: Text('Go back!'),
              ),
              CountdownWidget(10),
            ])));
  }
}

class CountdownWidget extends StatefulWidget {
  final int _countdown;
  
  CountdownWidget(this._countdown);
  
  @override
  _CountdownWidgetState createState() => _CountdownWidgetState(this._countdown);
}

class _CountdownWidgetState extends State<CountdownWidget> {
  Timer _timer;
  int _countdown;
  
  _CountdownWidgetState(this._countdown);

  @override
  void initState() {
    _timer = Timer.periodic(Duration(seconds: 1), (timer) {
      if (_countdown <= 0) {
        timer.cancel();
        return;
      }
      setState(() {
        _countdown--;
      });
    });
    super.initState();
  }

  @override
  void dispose() {
    _timer?.cancel();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Text(_countdown.toString());
  }
}