[firebase_admob]Destroy adView Before contentView Checking

Author: collinjackson

packages/firebase_admob/android/src/main/java/io/flutter/plugins/firebaseadmob/MobileAd.java

@@ -177,11 +177,11 @@ void show() {
     void dispose() {
       super.dispose();
 
+      adView.destroy();
+
       View contentView = activity.findViewById(id);
       if (contentView == null || !(contentView.getParent() instanceof ViewGroup)) return;
 
-      adView.destroy();
-
       ViewGroup contentParent = (ViewGroup) (contentView.getParent());
       contentParent.removeView(contentView);
     }