[FIX] point_of_sale: error with multi picking validation

before this commit, on validating multiple picking
from the transfers tree view is raising singleton
exception.

* enable "Ship Later" feature in pos.
* create two orders with ship later.
* from inventory app, select the above two
pickings are from action button click validate
* exception is raised.

after this commit, no traceback wont be shown.

closes odoo/odoo#120787

Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>