An AJAX HTTP Error message in VIEWS after upgrade to Drupal 7.12

I got the error after upgrading from 7.10 to 7.12. It happens when am working on Views.
Am using

Webform ver: 7.x-3.15
Drupal ver: 7.12

An AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: /admin/structure/views/view/users/preview/page/ajax
StatusText: OK
ResponseText: 
Fatal error:  Call to a member function execute() on a non-object in /home1/mywebsite/public_html/website/sites/all/modules/webform/views/webform_handler_field_submission_count.inc on line 55