Promotion does not display warning message when no shipping method selected

Description

Steps to reproduce:

1. Prepare a Campaign applied for all markets.
2. Create a shipping promotion that has "shipping method" properties in that campaign.
3. Check a shipping method for the USA.
4. Update a Campaign applied for Brazil or move the discount to another Campaign applied for Brazil.
5. Edit the promotion => Only shipping methods for Brazil are displayed and all are unchecked.
6. Update the discount's description and Save.

Expected:
Cannot save. Should show warning message "At least one shipping method must be selected."

Actual:
Update is successfully saved.