Bugfix cant call method parent

Fixes for two bugs discovered with plugin installed on mt.org production web site:
- The commit to display a custom field if the field is defined as a system-wide field addresses the error 'Can't call method "parent_id" on an undefined value' on the listing page for the object the custom field was defined for. Example: Custom field defined for Pages at system overview level; error then occurs when browsing to Manage Pages page at website level.  Note: This commit results in system-wide custom fields being displayed at blog and website levels.
- The commit to display a custom field at blog/website level addresses failure to display custom fields defined at the website level on the listing page for the object at the website level.  Example: Custom field defined for Pages at website level; field is not displayed on Manage Pages page at website level.