Email Receipt error in POS v13.16.1

My email works normal, able to send and receive from Erpnext, but can not send email receipt from POS
ERPNext: v13.16.1
Frappe Framework: v13.16.0

frappe.core.doctype.communication.email.sendmail


Traceback (most recent call last):
  File "/home/frappe/frappe-bench/apps/frappe/frappe/utils/background_jobs.py", line 104, in execute_job
    method(**kwargs)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/core/doctype/communication/email.py", line 462, in sendmail
    recipients=recipients, cc=cc, bcc=bcc)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/core/doctype/communication/communication.py", line 211, in _notify
    _notify(self, print_html, print_format, attachments, recipients, cc, bcc)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/core/doctype/communication/email.py", line 151, in _notify
    prepare_to_notify(doc, print_html, print_format, attachments)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/core/doctype/communication/email.py", line 231, in prepare_to_notify
    doc.content += get_attach_link(doc, print_format)
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'str'

Please update your erpnext and frappe to 13.17.0 and 13.17.1 respectively and check if the issue still persist.

unfortunately i can not update my erpnext and frappe since i use frappecloud service, so the issue still exist

it’s been updated to
ERPNext: v13.17.0
Frappe Framework: v13.17.1
but the issue still persist
can anyone help?