[FIX] mail: check existence of record in FollowerListMenu

This commit fixes a race condition that makes several tours crash.
The reason is that the underlying record is removed from the model
which produces a re-rendering. At the end of that re-rendering,
a component is unmounted. At that point, we remove an handler
relying on the existence of the record. As a consequence, there
was a short time window during which the handler would crash if it
was called.

Part-of: odoo/odoo#92475

addons/mail/static/src/components/follower_list_menu/follower_list_menu.js

@@ -57,6 +57,9 @@ export class FollowerListMenu extends Component {
      * @param {MouseEvent} ev
      */
     _onClickCaptureGlobal(ev) {
+        if (!this.followerListMenuView.exists()) {
+            return;
+        }
         // since dropdown is conditionally shown based on state, dropdownRef can be null
         if (this._dropdownRef.el && !this._dropdownRef.el.contains(ev.target)) {
             this.followerListMenuView.hide();