fix: Fixes undefined error.

modules/xmpp/ChatRoom.js

 
             // there is no point of sending conference iq when in visitor mode
             const preJoin
-                = this.options.disableFocus || this.options.hosts.visitorFocus
+                = this.options.disableFocus || this.options.hosts?.visitorFocus
                     ? Promise.resolve()
                     : this.moderator.allocateConferenceFocus();
 
-            if (this.options.hosts.visitorFocus) {
+            if (this.options.hosts?.visitorFocus) {
                 this.moderator.setFocusUserJid(this.options.hosts.visitorFocus);
             }