java - 关闭全屏广告时的android admob顶部广告请求

Question

当插页式广告（全屏广告）被关闭时，我需要启动热门广告请求。为Top Ad设置 adlistener 时代码崩溃。我该如何解决？谢谢你。

Intersticial is requested on onCreate,
interstitial = new InterstitialAd(this, ID_for_my_App);
AdRequest adRequest = new AdRequest();
interstitial.loadAd(adRequest);
interstitial.setAdListener(this);  

关闭时，应请求Top Ad但崩溃，

 @Override
      public void onDismissScreen(Ad ad) 
      {
        interstitial.setAdListener(null);
         adMob.setAdListener(this);
         AdRequest adRequest = new AdRequest();
         adMob.loadAd(adRequest);
      }