[FIX] website_event: event registration in edit mode

The event registration form is made up of a public widget that
instantiates an internal normal widget. All public widget are
automatically destroyed when entering in edit mode as well as the widget
it created. Therefore, it's main element is removed from the DOM when
the widget is destroyed. Here, we're detaching the main element from the
internal widget before the widget is destroyed so that we avoid to
destroy his main element from the DOM when entering in edit mode.

task-2210733

closes odoo/odoo#48215

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/website_event/static/src/js/website_event.js

@@ -89,8 +89,16 @@ publicWidget.registry.EventRegistrationFormInstance = publicWidget.Widget.extend
      */
     start: function () {
         var def = this._super.apply(this, arguments);
-        var instance = new EventRegistrationForm(this);
-        return Promise.all([def, instance.attachTo(this.$el)]);
+        this.instance = new EventRegistrationForm(this);
+        return Promise.all([def, this.instance.attachTo(this.$el)]);
+    },
+    /**
+     * @override
+     */
+    destroy: function () {
+        this.instance.setElement(null);
+        this._super.apply(this, arguments);
+        this.instance.setElement(this.$el);
     },
 });