[18.0][FIX] sequence_check_digit: fix ValueError: Expected singleton

[Ricardoalso]

was causing a ValueError: Expected singleton

Traceback (most recent call last):
  File "/odoo/external-src/server-ux/sequence_check_digit/models/ir_sequence.py", line 38, in check_check_digit_formula
    self.get_next_char(0)
  File "/odoo/external-src/server-ux/sequence_check_digit/models/ir_sequence.py", line 66, in get_next_char
    self.ensure_one()
  File "/odoo/src/odoo/models.py", line 6212, in ensure_one
    raise ValueError("Expected singleton: %s" % self)
ValueError: Expected singleton: ir.sequence(12, 13)

[OCA-git-bot]

This PR has the approved label and has been created more than 5 days ago. It should therefore be ready to merge by a maintainer (or a PSC member if the concerned addon has no declared maintainer). 🤖

[etobella]

/ocabot merge patch

[OCA-git-bot]

Hey, thanks for contributing! Proceeding to merge this for you.
Prepared branch 18.0-ocabot-merge-pr-1014-by-etobella-bump-patch, awaiting test results.

[OCA-git-bot]

Congratulations, your PR was merged at c9085db. Thanks a lot for contributing to OCA. ❤️