AdvMulti-Select HTML field type is deprecated and removed in CiviCRM 5.3 (See here). Thus, within the code for filling multi-value custom fields, the API call for retrieving custom fields will always fail with 'AdvMulti-Select' is not a valid option for field html_type API error.
AdvMulti-Select
HTML field type is deprecated and removed in CiviCRM 5.3 (See here). Thus, within the code for filling multi-value custom fields, the API call for retrieving custom fields will always fail with'AdvMulti-Select' is not a valid option for field html_type
API error.