Okay, found the event causing this error.
Basically, I had a function call with string parameter. In the called function, there was another function call with Function.Param(0). I expected the string parameter to be taken.
When I removed the parameter and had the function instead refer the global variable instead, the error no longer pops up.
Hope that helps, —
It was like this (with Variable at "Something"):
Event1: Call "SetStuff",Variable
On "SetStuff": Call "DoStuff",Function.Param(0)
This threw the this.eventvar is null error, for some reason.