[FIX] mail: race condition in mail composer

When an user sends an email from the chatter then quickly switch to
another record, it may happen that this.composer is already destroyed.

We prevent to display a traceback with this fix.

addons/mail/static/src/js/chatter.js

@@ -919,7 +919,7 @@ var Chatter = form_common.AbstractField.extend({
         this.mute_new_message_button(true);
     },
     close_composer: function (force) {
-        if (this.composer.is_empty() || force) {
+        if (this.composer && (this.composer.is_empty() || force)) {
             this.composer.do_hide();
             this.composer.$input.val('');
             this.mute_new_message_button(false);