Cron Preview in Edit Mode, throws errors while editing

Cron in sidebar allows you to edit and real time be provided with preview of dates. While a user is typing errors can be thrown to the main window. Like this:

Errors should not be possible while mid-edit. It would be better that errors in cron always yield an empty preview and that it is not possible to Save changes to the cron unless it is a valid expression.

Note that the developerMessage returned by REST endpoint, generally provides more useful info: