public synchronized CompletableFuture<List<Packet>> getCallback() {
            return callback;
        }
The state of the federation join. Null means that the request is pending completion. True means a successful
join was achieved, while false means that the join request failed or was aborted.
xmppserver/src/main/java/org/jivesoftware/openfire/muc/spi/FMUCHandler.java