User can get a kernel 3 error on user/register page.

Below is the scenario :

• A user goes on user/register, but does not sign up.
  => It creates an internal draft.
  => A value "RegisterUserID" is set in session.

• The cronjob "internal_draft_cleanup" is launched periodically.
  => The draft is removed

• The user goes again on user/register with the same cookie. As the valude "RegisterUserID" is set in session, eZ tries to fetch the objet but it is not available.