This service searches for the primary email-address of the given party, if it doesn't have a preffered contact mechanism ID in its parameters.
For this it uses the service getPartyEmail which, as a result, can deliver an optional email-address.
This is where the problem is: the result is optional.
Without any further checks, the result, empty or not, will be used to call another service called createContactListPartyStatus. For this service the field preferredContactMechId is a non optional input value, which is set before from the result of getPartyEmail.
If the party email could not be found, because a party group is added, or simply because the party has no mail given for some reason, this service call will result in an error because of missing parameters.