[12.2] Why do I get a "handler not defined" error when using list operations?

List operations bring up this error when passed something that isn't a list. A function like GetAProp() is not defined for integers, for example - there's no possible right answer to the question "what's the fifth item in 1?"

A common cause of this error is not declaring lists as global. Lists only persist as long as the variables referring to them persist. If a list is defined in a local variable, and the handler it was defined in finishes, the list gets cleared away. If you then try to use it again without redefining it, you get this error.