Steps to reproduce:
1. Open host items list and select items for mass update.
2. Check "Update interval" on mass update form and delete "Delay" input field value.
3. Press submit button

Result:
User is redirected to items list with error message:
Field "Update interval" is not correct: a time unit is expected
Expected:
User stay in mass update form with same error message.

Thanks for report to iivs.