BugFix issue commons-app#2652 (commons-app#2706)

* Addded null check on bookmark before operating on it

Author: ashishkumar468

app/src/main/java/fr/free/nrw/commons/bookmarks/pictures/BookmarkPicturesDao.java

@@ -6,17 +6,14 @@
 import android.database.sqlite.SQLiteDatabase;
 import android.os.RemoteException;
 import android.support.annotation.NonNull;
-
+import fr.free.nrw.commons.bookmarks.Bookmark;
 import java.util.ArrayList;
 import java.util.List;
-
 import javax.inject.Inject;
 import javax.inject.Named;
 import javax.inject.Provider;
 import javax.inject.Singleton;
 
-import fr.free.nrw.commons.bookmarks.Bookmark;
-
 import static fr.free.nrw.commons.bookmarks.pictures.BookmarkPicturesContentProvider.BASE_URI;
 
 @Singleton
@@ -121,6 +118,10 @@ private void deleteBookmark(Bookmark bookmark) {
      * @return boolean : is bookmark in database ?
      */
     public boolean findBookmark(Bookmark bookmark) {
+        if (bookmark == null) {//Avoiding NPE's
+            return false;
+        }
+
         Cursor cursor = null;
         ContentProviderClient db = clientProvider.get();
         try {