Fix setting up client_gadget in StartSaleQuoteStep

Editing an existent sale with client status, different of solvent,
the after_client__content_changed() was called before the
client_gadget be completely created. Causing an error.
- Check if the client_gadget was created, before configure other widgets

Change-Id: I7d46e4772c20433aacbd9eb9f987119f3edc7852
(cherry picked from commit 96fa8cc8f2d3ed0d0bc933220a1aa8d49bce6f9b)