title: Another exception was thrown: 'package:flutter/src/widgets/navigator.dart': Failed assertion: line 2216 pos 12: '!_debugLocked': is not true
body: <p>My app was running good I just update the code in one file and I am receiving this error before that error every page was navigating perfectly and now all the pages are good to work instead of this page I am navigating to this page  from home page 
  Another exception was thrown: 'package:flutter/src/widgets/navigator.dart': Failed assertion: line 2216 pos 12: '!_debugLocked': is not true. 
  I do not know where is the issue now. </p>
  
  <pre><code>import 'package:custom_chewie/custom_chewie.dart';
  import 'package:flutter/cupertino.dart';
  import 'package:flutter/material.dart';
  import 'package:video_player/video_player.dart';
  import 'package:firebase_admob/firebase_admob.dart';
  
  class ChewieDemo extends StatefulWidget {
    final String title;
  
    ChewieDemo({this.title = 'Chewie Demo'});
  
    @override
    State&lt;StatefulWidget&gt; createState() {
      return new _ChewieDemoState();
    }
  }
  
  class _ChewieDemoState extends State&lt;ChewieDemo&gt; {
    int counter=0;
    static final MobileAdTargetingInfo targetingInfo = MobileAdTargetingInfo(
      keywords: ['Games', 'Puzzles'],
    );
  
    BannerAd bannerAd;
    InterstitialAd interstitialAd;
    RewardedVideoAd rewardedVideoAd;
  
    BannerAd buildBanner() {
      return BannerAd(
          adUnitId: BannerAd.testAdUnitId,
          size: AdSize.banner,
          listener: (MobileAdEvent event) {
            print(event);
          });
    }
  
    InterstitialAd buildInterstitial() {
      return InterstitialAd(
          adUnitId: InterstitialAd.testAdUnitId,
          targetingInfo: targetingInfo,
          listener: (MobileAdEvent event) {
            if (event == MobileAdEvent.failedToLoad) {
              interstitialAd..load();
            } else if (event == MobileAdEvent.closed) {
              interstitialAd = buildInterstitial()..load();
            }
            print(event);
          });
    }
    TargetPlatform _platform;
    VideoPlayerController _controller;
  
    @override
    void initState() {
      super.initState();
      _controller = new VideoPlayerController.network(
        'https://github.com/flutter/assets-for-api-docs/blob/master/assets/videos/butterfly.mp4?raw=true',
      );
      FirebaseAdMob.instance.initialize(appId: FirebaseAdMob.testAppId);
      bannerAd = buildBanner()..load();
      interstitialAd = buildInterstitial()..load();
    }
    @override
    void dispose(){
      super.dispose();
      _controller.dispose();
    }
  
    @override
    Widget build(BuildContext context) {
      bannerAd ..load()..show(
        anchorOffset: 20.0,
        anchorType: AnchorType.top,
      );
      Future&lt;bool&gt; _onBackPressed() {
        if(counter&lt;1){
          interstitialAd
            ..load()
            ..show();
          counter++;
        }
        else{bannerAd.dispose();
        Navigator.pop(context, true);
        }
  
      }
      return WillPopScope(
        child: Scaffold(
          appBar: new AppBar(
            title: new Text(widget.title),
          ),
          body: new Column(
            children: &lt;Widget&gt;[
              new Expanded(
                child: new Center(
                  child: new Chewie(
                    _controller,
                    aspectRatio: 3 / 2,
                    autoPlay: true,
                    looping: true,
                  ),
                ),
              ),
  
              new Row(
                children: &lt;Widget&gt;[
                  new Expanded(
                    child: new FlatButton(
                      onPressed: () {
                        setState(() {
                          _platform = TargetPlatform.android;
                        });
                      },
                      child: new Padding(
                        child: new Text("Android controls"),
                        padding: new EdgeInsets.symmetric(vertical: 16.0),
                      ),
                    ),
                  ),
                  new Expanded(
                    child: new FlatButton(
                      onPressed: () {
                        setState(() {
                          _platform = TargetPlatform.iOS;
                        });
                      },
                      child: new Padding(
                        padding: new EdgeInsets.symmetric(vertical: 16.0),
                        child: new Text("iOS controls"),
                      ),
                    ),
                  )
                ],
              )
            ],
          ),
        ),
        onWillPop: _onBackPressed,
      );
  
  
    }
  }
  </code></pre>