Unable to start activity ComponentInfo, on a null object reference

• Issues with Loading AdMob Ads on App
• Android google ads error (null reference ?)

Error

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.domain.sub/com.domain.sub.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.gms.ads.AdView.loadAd(com.google.android.gms.ads.AdRequest)' on a null object reference

해당 activity 에서 R.id.adView 를 찾지 못해서 생기는 문제로 R.id.adView 가 있어야 한다.