[fix] add condition to avoid unknown element

static/src/js/mail_message.js

@@ -23,6 +23,11 @@ odoo.define('cgscop_mail_message.MailMessage',function(require){
             var commands = this.options.commandsEnabled ?
                             this._mentionManager.getListenerSelection('/') :
                             [];
+            if (this.$('#oe_restricted_message').length) {
+                var is_restricted = this.$('#oe_restricted_message')[0].checked
+            } else {
+                var is_restricted = false
+            }
             return $.when({
                 content: this._mentionManager.generateLinks(value),
                 attachment_ids: _.pluck(this.get('attachment_ids'), 'id'),
@@ -30,7 +35,7 @@ odoo.define('cgscop_mail_message.MailMessage',function(require){
                 canned_response_ids: _.uniq(_.pluck(this._mentionManager.getListenerSelections()[':'], 'id')),
                 channel_ids: _.uniq(_.pluck(this._mentionManager.getListenerSelection('#'), 'id')),
                 command: commands.length > 0 ? commands[0].name : undefined,
-                restricted: this.$('#oe_restricted_message')[0].checked,
+                restricted: is_restricted
             });
         },