Entries and User fieldtypes should handle error and show warning when ID is not found

[mikemartin]

Bug Description

I've ran into this issue a couple times recently. It looks like when using a Relate field like Entries or Users, if the ID doesn't match an entry then I receive the error:

call to member function handle() on null

However, the Stack Selector seems to handle the missing ID more gracefully and shows warning on the field. Can we ensure that the Select modes show a warning too?