[FIX] Don't break inheritance chain

account_direct_debit/models/account_invoice.py

@@ -135,6 +135,7 @@ class AccountInvoice(orm.Model):
         Maybe apply a similar trick when overriding the buttons' 'states'
         attributes in the form view, manipulating the xml in fields_view_get().
         """
+        super(AccountInvoice, self)._register_hook(cr)
         self._columns['state'].selection.append(
             ('debit_denied', 'Debit denied'))