how to pop page after condition is true in flutter?

Question

I am trying to build a timer app where screen will pop after time become null the sample code is following

                              Padding(
                                padding: const EdgeInsets.all(8.0),
                                child: CountdownTimer(
                                    endTime: endTime,
                                    widgetBuilder: (_, CurrentRemainingTime time) {
                                      if (time == null) {
                                          Navigator.of(context).pop();

                                      } else {
                                        return Text(
                                            '${(time.hours == null) ? "00" : time.hours}:${(time.min == null) ? "00" : time.min}:${(time.sec == null) ? "00" : time.sec}');
                                      }
                                    }),
                       

error Shown As Follows

E/flutter (21479): [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: 'package:flutter/src/widgets/navigator.dart': Failed 
assertion: line 4334 pos 12: '<optimized out>': is not true.
E/flutter (21479): #0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:46:39)
E/flutter (21479): #1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:36:5)
E/flutter (21479): #2      NavigatorState._pushEntry (package:flutter/src/widgets/navigator.dart:4334:12)
E/flutter (21479): #3      NavigatorState.push (package:flutter/src/widgets/navigator.dart:4270:5)
E/flutter (21479): #4      showGeneralDialog (package:flutter/src/widgets/routes.dart:1835:66)
E/flutter (21479): #5      Alert.show (package:rflutter_alert/src/alert.dart:57:18)

Solution

You can copy paste run full code below
You can use addPostFrameCallback and return Container()
code snippet

widgetBuilder: (_, CurrentRemainingTime time) {
      if (time == null) {
        WidgetsBinding.instance.addPostFrameCallback((_) {
          Navigator.of(context).pop();
        });
        return Container();

working demo

full code

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

void main() {
  runApp(MyApp());
}

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

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("test"),
        ),
        body: Center(
          child: RaisedButton(
            child: Text('To CountDown page'),
            onPressed: () {
              Navigator.push(
                context,
                MaterialPageRoute(builder: (context) => CountDownPage()),
              );
            },
          ),
        ));
  }
}

class CountDownPage extends StatefulWidget {
  CountDownPage({Key key}) : super(key: key);

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

class _CountDownPageState extends State<CountDownPage> {
  int endTime = DateTime.now().millisecondsSinceEpoch + 1000 * 3;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("count down"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Padding(
                padding: const EdgeInsets.all(8.0),
                child: CountdownTimer(
                    endTime: endTime,
                    widgetBuilder: (_, CurrentRemainingTime time) {
                      if (time == null) {
                        WidgetsBinding.instance.addPostFrameCallback((_) {
                          Navigator.of(context).pop();
                        });
                        return Container();
                      } else {
                        return Text(
                            '${(time.hours == null) ? "00" : time.hours}:${(time.min == null) ? "00" : time.min}:${(time.sec == null) ? "00" : time.sec}');
                      }
                    })),
          ],
        ),
      ),
    );
  }
}