The error is due to call to the protected function getRegistrationContactID()getRegistrationContactID()
in CRM_Event_Form_Registration_AdditionalParticipantCRM_Event_Form_Registration_AdditionalParticipant
class for multiple evenevent participants registration.
The issue got resolved in my installation (civicrm 4.7.19) after changing the access control of the function getRegistrationContactID() form protectedgetRegistrationContactID()
from protected
to publicpublic
.
The file location is, civicrm-module/CRM/Event/Form/Registration.php
For getRegistrationContactID function definition, replace protected with public. civicrm-module/CRM/Event/Form/Registration.php