fix(JitsiConference): fix crash on this.room if the room was disposed

JitsiConference.js

@@ -1298,10 +1298,12 @@ JitsiConference.prototype.sendApplicationLog = function(message) {
  * Checks if the user identified by given <tt>mucJid</tt> is the conference
  * focus.
  * @param mucJid the full MUC address of the user to be checked.
- * @returns {boolean} <tt>true</tt> if MUC user is the conference focus.
+ * @returns {boolean|null} <tt>true</tt> if MUC user is the conference focus,
+ * <tt>false</tt> when is not. <tt>null</tt> if we're not in the MUC anymore and
+ * are unable to figure out the status or if given <tt>mucJid</tt> is invalid.
  */
 JitsiConference.prototype._isFocus = function (mucJid) {
-    return this.room.isFocus(mucJid);
+    return this.room ? this.room.isFocus(mucJid) : null;
 };
 
 /**

modules/xmpp/ChatRoom.js

@@ -669,7 +669,9 @@ ChatRoom.prototype.removePresenceListener = function (name) {
  * Checks if the user identified by given <tt>mucJid</tt> is the conference
  * focus.
  * @param mucJid the full MUC address of the user to be checked.
- * @returns {boolean} <tt>true</tt> if MUC user is the conference focus.
+ * @returns {boolean|null} <tt>true</tt> if MUC user is the conference focus or
+ * <tt>false</tt> if is not. When given <tt>mucJid</tt> does not exist in
+ * the MUC then <tt>null</tt> is returned.
  */
 ChatRoom.prototype.isFocus = function (mucJid) {
     var member = this.members[mucJid];