Fixed an NPE while changing a DateField from 24 to 12h clock (#13722)

A client-side NullPointerException was fixed by not reusing
the existing VTime panel and always recreating it instead.
Otherwise the ListBox for switching between AM/PM might have
been null.

Change-Id: I8d54d91627043a12b52ac5d5e54d6f7a729af1ac