I used to develop the use case to implement the SMAX2SMAX Case exchange for the Incident module. 

Getting the error when I tried to push the below fields and the field values were pushed as 'null' even though values were selected during the incident creation.

 KEY                                                                                              Value

ContactPerson.Upn                                                           SourceIncident.Properties.ContactPerson,

OwnedByPerson.Upn                                                        SourceIncident.Properties.OwnedByPerson, 

RegisteredForServiceComponent.DisplayLabel            SourceIncident.Properties.RegisteredForServiceComponent.