* If an undefined group is referenced.
*/
private List<Group> getSelectedGroups(List<String> groupNames, RuleSet ruleSet) throws RuleSetResolverException {
final List<Group> selectedGroups = new ArrayList<>();
for (String groupName : groupNames) {
Group group = ruleSet.getGroups().get(groupName);
if (group != null) {
selectedGroups.add(group);
} else {
LOGGER.warn("Group '{}' is not defined, skipping.", groupName);
}