Date Format shouldn't be mandatory on a Date range widget

I followed the how-to How to Configure a Search Filter With Facets and Other Aggregates. After I add and configure the Date range aggregate widget, I get an validation error about format:

On search layout, widget 'Container Creation Date' subwidgets, widget 'Date Range (created)': date aggregate widget 'created' must have a format set.

But format didn't used to be mandatory and shouldn't be.