Illegal choice for display block

Problem/Motivation

Had an interesting error occur while creating a digest. Before adding a block to display I changed the schedule and it produced a form error saying there was an illegal choice for the block and this error was displayed just above the cron translation.

Steps to reproduce

1. Create a new digest
2. Before choosing a display, update the schedule to a valid option
3. Allow the translation to be displayed.
4. Change the cron to another valid option. Then un focus the field.
5. The block display error should appear.

Proposed resolution

It seems to be that the form is being validated at the wrong time when the ajax loads the translation. Likely a simple check here would suffice to suppress the block warning.