Library crashing in Nougat

[Zulqurnain]

library is crashing in QMobile S8 (QMobile_S8), 2048MB RAM, Android 7.0

java.lang.IllegalStateException: 
  at com.facebook.ads.internal.DisplayAdController.b (Unknown Source)
  at com.facebook.ads.InterstitialAd.show (Unknown Source)
  at kr.docs.smartad.SmartAdInterstitial.showLoadedAd (SmartAdInterstitial.java:61)
  at kr.docs.smartad.SmartAdInterstitial.onAdLoaded (SmartAdInterstitial.java:175)
  at com.facebook.ads.InterstitialAd$1.a (Unknown Source)
  at com.facebook.ads.internal.DisplayAdController$11.onInterstitialAdLoaded (Unknown Source)
  at com.facebook.ads.internal.adapters.m$2.c (Unknown Source)
  at com.facebook.ads.internal.adapters.m$2.a (Unknown Source)
  at com.facebook.ads.internal.d.b$1$2.run (Unknown Source)
  at android.os.Handler.handleCallback (Handler.java:836)
  at android.os.Handler.dispatchMessage (Handler.java:103)
  at android.os.Looper.loop (Looper.java:203)
  at android.app.ActivityThread.main (ActivityThread.java:6251)
  at java.lang.reflect.Method.invoke (Native Method)
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run (ZygoteInit.java:1063)
  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:924)