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, Ashley.
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.