The error you are getting is due to your attempting to perform your Fragment manipulation AFTER #onSaveInstanceState has been called on your Activity. Ie your Activity has been stopped and possibly destroyed, undoubtedly because the Ad Activity has been displayed. When the Ad is closed your Activity will be restarted (possibly even recreated), so do whatever UI manipulation you want at that point.
On a side note:
DO NOT call displayInterstitial from AdListener#onAdLoaded.
This will provide a really poor user experience as well as cause you mountains of problems with ads showing up when you least expect them such as after existing your Activity.
Instead call DisplayInterstitial at a natural break point in your app.