setting required on missing fields causes fatal error

[CaptainN]

If you call set_required on a once form (such as in a validation routine) and the form element is missing, OnceForm will generate a cryptic error along the lines of "Fatal error: Call to a member function required() on a non-object in /MyForm.php on line 327".

The system should probably throw up a warning instead or a catchable error with a more descriptive message in a suitable mode.