Fix memory leak in CategoryDetailsActivity (commons-app#1792)

Author: ashishkumar468

app/src/main/java/fr/free/nrw/commons/category/CategoryImagesListFragment.java

@@ -263,10 +263,7 @@ private void handleSuccess(List<Media> collection) {
      * @return  GridView Adapter
      */
     public ListAdapter getAdapter() {
-        if(gridView == null) {
-            return null;
-        }
-        return gridView.getAdapter();
+        return gridAdapter;
     }
 
 }