Custom Date Formats - problem undefined

[roberthurkala]

Hello,
I have a problem with Custom Date Formats (https://getdatepicker.com/6/plugins/customDateFormat.html)
I choose a date from my calendar, then mark it and delete it from input. After deletion, the following text is displayed: undefined.

This is problematic with forms when someone wants to delete the date.

Can it be fixed?

[Eonasdan]

please follow #2693