code: protected String getSessionIndex(final Assertion subjectAssertion) {
          List<AuthnStatement> authnStatements = subjectAssertion.getAuthnStatements();
          if (authnStatements != null && authnStatements.size() > 0) {
              AuthnStatement statement = authnStatements.get(0);
              if (statement != null) {
                  return statement.getSessionIndex();
              }
          }
          return null;
      }
docs: Searches the sessionIndex in the assertion
  
  @param subjectAssertion assertion from the response
  @return the sessionIndex if found in the assertion