Lookup "plus" feature throws error

This line causes an error when you choose to have the Lookup with Lookup::plus = true:

https://github.com/atk4/ui/blob/3cb026a3649efc1841d5d73a7608145da6212971/src/Form/Control/Lookup.php#L257

Error is "Atk4\Data\Exception: Expected model, but instance is an entity".
