Steps to reproduce the issue

1. New 3.8.0 installation, no test data

2. In Global Config, turn on maximum error messages

3. Create a category, call it 'Test'

4. Create custom field and assign it to the category 'Test'

5. Go to Articles, set the list filter to filter on Category 'Test'

6. Click New Article, get error

   Notice: Array to string conversion in C:\xampp7\htdocs\test\administrator\components\com_fields\helpers\fields.php on line 330

Expected result

Actual result

Error as stated

System information (as much as possible)

Both live Siteground host and local xampp.
PHP7
J3.8.0

Additional comments

The error happens because the Category Filter dropdown has changed to multiselect, so in my example, the field helper is given an array of cat id's instead of a single cat id to work with. The New Article form tries to load custom fields for the specified category but its now an array of one or more catid's instead of just a single catid.

My suggestion would be to check the incoming catid for an array and if found, to just use the first catid.