The AvailableRealmsCallback should not result in a NPE if there is no mechanism configuration.

                } else if (callback instanceof AvailableRealmsCallback) {
                    Collection<String> names = stateRef.get().getMechanismConfiguration().getMechanismRealmNames();
                    if (log.isTraceEnabled()) {
                        log.tracef("Handling AvailableRealmsCallback: realms = [%s]", String.join(", ", names));
                    }
                    if (! names.isEmpty()) {
                        ((AvailableRealmsCallback) callback).setRealmNames(names.toArray(new String[names.size()]));
                    }
                    handleOne(callbacks, idx + 1);