Re-use current connection's domain as ping destination instead of accessing config.hosts.domain directly.

modules/xmpp/xmpp.js

                 console.info("My Jabber ID: " + connection.jid);
 
                 // Schedule ping ?
-                var pingJid = config.hosts.domain;
+                var pingJid = connection.domain;
                 connection.ping.hasPingSupport(
                     pingJid,
                     function (hasPing) {