Do not display delete link if the relation type is stored in export

I have the relation types stored in exports, with the help of hook_ctools_plugin_api() and hook_relation_default_relation_types().

The problem is when I try to click the delete link on admin/structure/relation page:
Fatal error: Unsupported operand types in entity/includes/entity.property.inc on line 88

solution examples:
- quicktabs module does not display the delete link for exportables
- flag module makes the flag disabled when the delete link is clicked