Hide Listbox Editable Fields On The Form
If a form contains a listbox with editable fields by listbox_property, those
fields should be made hidden in the form by moving them in the "hidden" group.
Fields in hidden groups are not renderred at all (this is different from checking "hidden" property on field, in this case it's rendered as a
<input type='hidden'/>). If the hidden group does not exist in the form, you should create it.