Undefined variable: active_field in registration_date_form_registration_entity_settings_form_alter

Environment:
Drupal core 7.14
Date 7.x-2.5
Entity Registrations 7.x-1.0-beta2
Registration Date 7.x-1.0-alpha0

In oder to have the date Popup Picker for Open/Close registration dates I installed this module.

As soon as I want to edit the Registration (registrations/settings) I get the following Notice and the
date Picker doesn't appear.
Undefined variable: active_field in registration_date_form_registration_entity_settings_form_alter() (line 137 von /home/www/vtad7test/sites/all/modules/registration_date/registration_date.module).