Flutter countdown widget in StatefulBuilder. Will receive exception if user go back before countdown finished.

main.dart

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

void main() {
  runApp(MaterialApp(title: 'Timer in StatefulBuilder', 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(),
            ])));
  }
}

class CountdownWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    int countdown = 10;
    return StatefulBuilder(builder: (context, setState) {
      if (countdown > 0) {
        Timer(Duration(seconds: 1), () {
          setState(() {
            countdown--;
          });
        });
      }
      return Text(countdown.toString());
    });
  }
}