Calling `_instances` on a Property should give a clean error

(Bug, Closed -> Fixed, Priority: Critical, Test Status: Unit tested , Reported By Bad Link: Thing 3y284oe not found, )
Functions
Summary:
From Eric:
I recently tried to get
Release -> _instances -> _sort(....)
...and it hung on me. Eventually I figured out it was because Release was a Property, with Releases being the model.
Is this incorrect syntax that should error? Or is PROP NAME -> _instances valid?
While this is legal syntax, it makes no sense semantically. I suspect the "hang" was an internal error that didn't display properly. (Which also needs investigation.) _instances should check the type that it is being called on, and produce a clean error if it makes no sense.