hr_employee_ppe: error when assigning multiple PPE to an employee.

[azucenatrey]

Module

hr_employee_ppe

Describe the bug

When you assign multiple EPIs to an employee, the following error is returned:
'''
Error while validating constraint
Expected singleton: hr.employee.ppe(5, 6)
'''

To Reproduce

Affected versions: 12.0

Steps to reproduce the behavior:

1. Step in employee form.
2. Edit and add multiple PPEs.
3. Save.

Expected behavior
Save without error.

[azucenatrey]

Solution proposed in PR: #1351