我正在测试奖励视频广告功能，它似乎工作正常，直到我遇到异常，视频广告似乎显示得很好，但有时它根本没有显示！视频广告已加载，并且
onRewardedVideoAdFailedToLoad
根本没有触发，这里是例外：
07-21 20:48:31.710 26818-26818/ma.altaiir.testapp I/Ads: Starting ad request.
07-21 20:48:31.710 26818-26818/ma.altaiir.testapp I/Ads: This request is sent from a test device.
07-21 20:48:32.320 26818-26818/ma.altaiir.testapp I/Ads: Ad finished loading.
07-21 20:49:16.360 26818-26818/ma.altaiir.testapp W/Ads: Could not show rewarded video ad from adapter.
java.lang.NullPointerException: Attempt to read from field 'com.google.android.gms.ads.internal.client.bo ib.a' on a null object reference
at com.google.ads.mediation.AbstractAdViewAdapter.showVideo(:com.google.android.gms.DynamiteModulesA:228)
at com.google.android.gms.ads.internal.mediation.client.u.f(:com.google.android.gms.DynamiteModulesA:193)
at com.google.android.gms.ads.internal.reward.b.a(:com.google.android.gms.DynamiteModulesA:29)
at com.google.android.gms.ads.internal.reward.client.e.onTransact(:com.google.android.gms.DynamiteModulesA:16)
at android.os.Binder.transact(Binder.java:387)
at com.google.android.gms.internal.zznr$zza$zza.show(Unknown Source)
at com.google.android.gms.internal.zznz.show(Unknown Source)
at ma.altaiir.testapp.RewardedVideoAdHelper.show(RewardedVideoAdHelper.java:63)
at ma.altaiir.testapp.MainActivity$6.onClick(MainActivity.java:526)
at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:174)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:7325)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
07-21 20:49:16.360 26818-26818/ma.altaiir.testapp W/Ads: Could not call showVideo.
android.os.RemoteException
at com.google.android.gms.ads.internal.mediation.client.u.f(:com.google.android.gms.DynamiteModulesA:197)
at com.google.android.gms.ads.internal.reward.b.a(:com.google.android.gms.DynamiteModulesA:29)
at com.google.android.gms.ads.internal.reward.client.e.onTransact(:com.google.android.gms.DynamiteModulesA:16)
at android.os.Binder.transact(Binder.java:387)
at com.google.android.gms.internal.zznr$zza$zza.show(Unknown Source)
at com.google.android.gms.internal.zznz.show(Unknown Source)
at ma.altaiir.testapp.RewardedVideoAdHelper.show(RewardedVideoAdHelper.java:63)
at ma.altaiir.testapp.MainActivity$6.onClick(MainActivity.java:526)
at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:174)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:7325)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
07-21 20:49:22.040 26818-26818/ma.altaiir.testapp I/Ads: Starting ad request.
07-21 20:49:22.040 26818-26818/ma.altaiir.testapp I/Ads: This request is sent from a test device.
07-21 20:49:23.130 26818-26818/ma.altaiir.testapp W/Ads: The webview is destroyed. Ignoring action.
07-21 20:49:23.375 26818-26818/ma.altaiir.testapp I/Ads: Scheduling ad refresh 60000 milliseconds from now.
07-21 20:49:23.410 26818-26818/ma.altaiir.testapp I/Ads: Ad finished loading.
07-21 20:50:23.405 26818-26818/ma.altaiir.testapp I/Ads: Starting ad request.
07-21 20:50:23.405 26818-26818/ma.altaiir.testapp I/Ads: This request is sent from a test device.
07-21 20:50:23.970 26818-26818/ma.altaiir.testapp W/Ads: The webview is destroyed. Ignoring action.
07-21 20:50:24.160 26818-26818/ma.altaiir.testapp I/Ads: Scheduling ad refresh 60000 milliseconds from now.
07-21 20:50:24.185 26818-26818/ma.altaiir.testapp I/Ads: Ad finished loading.