The ‘Instantiated signal’ field for a Signal-type Event must be defined, whereas the ‘Called operation’ and ‘Expression’ fields must be empty.
An Event is the specification of a specific occurrence at a specific point in space and time.
A Signal-type Event is meant to define a Signal, and cannot trigger an Operation at the same time.
Add a reference to a Signal, if the Event doesn’t have one.
An Event referring to an Operation should be a Call-type Event.