Hi,
Sure! It looks like it is coming from the function which uses Advanced Custom Fields to get the field choices for the front-end filters. That function should return the field object, but it seems like for some reason it fails in some cases.
Nevertheless, I think it can be corrected very easily.
– Open up the wp-content/plugins/ajax-search-pro/includes/functions/functions.php file on your server
– Replace it’s contents with this.
– Save, and the error should go away immediately
If you need assistance with that, let me know.