code: @Override
      protected boolean isValidFragment(String fragmentName) {
  
          Boolean knownFrag = false;
  
          for (Class<?> cls : INNER_CLASSES) {
  
              if ( cls.getName().equals(fragmentName) ){
  
                  knownFrag = true;
  
                  break;
              }
          }
  
          return knownFrag;
      }
docs: Google found a 'security vulnerability' and imposed this hack.
  Have to check this fragment was actually conceived by this activity.