Bugfix. Add id to outgoing MUC messages

src/converse-chatview.js

@@ -823,7 +823,8 @@ converse.plugins.add('converse-chatview', {
              * @param { object } message - The message Backbone object that was added.
              */
             async onMessageAdded (message) {
-                if (this.get(message.get('id'))) {
+                const id = message.get('id');
+                if (id && this.get(id)) {
                     // We already have a view for this message
                     return;
                 }

src/headless/converse-muc.js

@@ -448,6 +448,7 @@ converse.plugins.add('converse-muc', {
                 const origin_id = _converse.connection.getUniqueId();
 
                 return this.addOccupantData({
+                    'id': origin_id,
                     'msgid': origin_id,
                     'origin_id': origin_id,
                     'from': `${this.get('jid')}/${this.get('nick')}`,