code: protected PermissionModel getPermission(int index) {
          if (permissions().isEmpty()) return null;
          if ((index) <= permissions().size()) {// avoid accessing index does not exists.
              return permissions().get(index);
          }
          return null;
      }
docs: return PermissionModel at specific index.
  <p/>
  if index > {@link #permissions().size()} null will be returned