Hey i am trying to send the result of survey to employee but i am getting the singleton error but i am already using for loops. Kindly have a look at my code

def _prepare_result_receivers(self):
    """
    @return: returns a list of tuple (survey, list of employees).
    """
    appraisal_send_to = []
    if self.emp_appr:
        appraisal_send_to.append((self.survey_id, self.emp_appr))
    return appraisal_send_to

@api.multi
def throw_appraisal(self):
    ComposeMessage = self.env['survey.mail.compose.message']
    for appraisal in self:
        appraisal_send_to = appraisal._prepare_result_receivers()
        for survey,receivers in appraisal_send_to:
            for employee in receivers:
                email = employee.related_partner_id.email or employee.work_email
                if not email:
                    continue
                render_template = appraisal.result_template_id.with_context(email=email, survey=survey, employee=employee).generate_email([appraisal.id])
                values = {
                    'survey_id': survey.id,
                    'public': 'email_private',
                    'partner_ids': employee.related_partner_id and [(4, employee.related_partner_id.id)] or False,
                    'multi_email': email,
                    'subject': '%s appraisal: %s' % (appraisal.employee_id.name, survey.title),
                    'body': render_template[appraisal.id]['body'],
                    'date_deadline': appraisal.date_close,
                    'model': appraisal._name,
                    'res_id': appraisal.id,
                }
                compose_message_wizard = ComposeMessage.with_context(active_id=appraisal.id, active_model=appraisal._name, notif_layout="mail.mail_notification_light").create(values)
                compose_message_wizard.send_mail()  # Sends a mail and creates a survey.user_input
                if employee.user_id:
                    user_input = survey.user_input_ids.filtered(
                        lambda user_input: user_input.partner_id == employee.user_id.partner_id and user_input.appraisal_id == appraisal and user_input.state != 'done'
                    )
                    if user_input:
                        form_url = survey.print_url + '/' + user_input[0].token
                    else:
                        form_url = survey.print_url
                    appraisal.activity_schedule(
                        'hr_appraisal.mail_act_appraisal_form', appraisal.date_close,
                        note=_('Fill form <a href="%s">%s</a> for <a href="#" data-oe-model="%s" data-oe-id="%s">%s\'s</a> appraisal') % (
                            form_url, survey.display_name,
                            appraisal.employee_id._name, appraisal.employee_id.id, appraisal.employee_id.display_name),
                        user_id=employee.user_id.id)
        appraisal.message_post(body=_("Appraisal form(s) have been sent"))
    return True

And here is the error i am getting
  File "/opt/odoo12/odoo-custom-addons/dos_appraisal_send/models/send_appr.py", line 33, in throw_appraisal
    render_template = appraisal.result_template_id.with_context(email=email, survey=survey, employee=employee).generate_email([])
  File "/opt/odoo12/odoo/addons/mail/models/mail_template.py", line 422, in generate_email
    self.ensure_one()
  File "/opt/odoo12/odoo/odoo/models.py", line 4764, in ensure_one
    raise ValueError("Expected singleton: %s" % self)
ValueError: Expected singleton: mail.template()