As discussed in the thread Errors While creating an Email Activity?
with the 1402 release of the studio, you cannot create an email inside an event of the activity business object, because of an issue on the backend. SAP will provide an hotfix but for now you can solve the problem with a workaround:
1) First of all create a new custom BO with an action modeled as following
The bo will receive the email request and will create the email instance.
2) Create the webservice based on the modeled BO, create the Service Integration, the Communication Scenario, the Communication system and the Communication arrangement
3) Switch the call in the activity extension!
The WS processing happens in a different dialog step/database transaction of the original activity BO so you will not get the issue anymore