title: Flutter firebase_admob banner is still shown on every screen
body: <p>I'm using firebase_admob package in a flutter project, and when I load and show a banner ad on the first screen it's loaded and displayed correctly, but when I navigate to a new screen that doesn't contain a banner, the old banner is still shown on the new screen,</p>
  
  <p>this is the creation code of the banner called in the initState()</p>
  
  <pre><code>BannerAd createBannerAd() {
      return BannerAd(
          adUnitId: BannerAd.testAdUnitId,
          //Change BannerAd adUnitId with Admob ID
          size: AdSize.smartBanner,
          targetingInfo: AdsUtil.targetingInfo,
          listener: (MobileAdEvent event) {
            if (event == MobileAdEvent.loaded) {
              // dispose after you received the loaded event seems okay.
              if (mounted) {
                _bannerAd..show();
                setState(() {
                  isBannerShown = true;
                });
              } else {
                _bannerAd = null;
                setState(() {
                  isBannerShown = false;
                });
              }
            } else if (event == MobileAdEvent.failedToLoad) {
              _bannerAd = null;
              setState(() {
                isBannerShown = false;
              });
            }
            //  print("********** ********** BannerAd $event");
          });
    }
  @override
    void dispose() {
      _bannerAd.dispose();
      super.dispose();
    }
  </code></pre>
  
  <p>Navigation to the second screen code : </p>
  
  <pre><code>    IconButton(
                      icon: Icon(Icons.settings),
                      onPressed: () {
                        Navigator.of(context).pushNamed(SettingsPage.routeName);
                      },
                    ),
  </code></pre>
  
  <p>I want to find a solution that when I navigate to a new screen the old banner get destroyed and not displayed in the new screen I navigated to.</p>
  
  <p>Thanks</p>