LPS-51186 test expected behaviour for event registry
LPS-51186 force a NPE on null listener

It doesn't make sense to try to register a null event listener, so it's
probably a caller error. Fail fast to make this situation easier to
detect and fix.
LPS-51186 test expected behaviour when firing events
LPS-51186 use "model" instead of "target" or "payload"

Use the same term everywhere. Use "model" instead of the alternatives to
make explicit its relation with the modelClass parameter.
LPS-51186 SF
LPS-51186 SF