How to call AdMob Interstitial Cocos2d-x WP8 whenever game over?

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Windows

phone

I've implemented by following this link "AdMob Interstitial Cocos2d-x WP8[^]". I called interstitial ad in gameover page and it works fine (Ad show ). Now if i tap on play again button and my game crash itself while game staring. I couldn't find out why this problem occurred and how to solve?

Note: If i navigate to any where outside from my game (for example: Tap on "more" button to go to store then come back to game play; same thing happened.