Invert the logic on this parameter path validity checker.

Renamed the method to isInvalidParameterPath() because that's
closer to what it does: flag a property path that is definitely
not valid. The way it was before, it looked like it alone could
tell you whether a path was valid or not, and it doesn't do that.
Immediately show if a parameter has an invalid path.

Made isInvalidParameterPath() public.
Don't make it easy to create a bogus path.
Found the bug with paths sometimes not updating visually.