Steps to reproduce the issue

use a module that uses the backend model with several categories

Expected result

there shouldn't be any notice

Actual result

there is a notice
Notice: Array to string conversion in /administrator/components/com_contact/models/contacts.php on line 134

System information (as much as possible)

joomla staging branch

Additional comments

it's just a samll inconsistency in backend models contact.
The filter category_id is expected to be a numeric or an array in getListQuery function, but the store id function assumes it's a string.
hence, setting the filter to an array triggers a warning:

Notice: Array to string conversion in /administrator/components/com_contact/models/contacts.php on line 134