Error in user/register

I am getting the following error when an unauthenticated visitor clicks on the /user/register link:

Warning: call_user_func() expects parameter 1 to be a valid callback, class 'Drupal\workbench_access\WorkbenchAccessManager' does not have a method 'getDefaultValue' in Drupal\Core\Field\FieldConfigBase->getDefaultValue() (line 396 of core/lib/Drupal/Core/Field/FieldConfigBase.php).

Drupal Version: 8.4.2
Workbench: Version: 8.x-1.0
Workbench_moderation: Version: 8.x-1.3
Workbench_access: Version: 8.x-1.0-alpha7