Fatal error after disabling Date Popup module

If one disables the Date Popup module but still has content types with fields that use the popup widget then one gets the

Fatal error: Call to undefined function date_popup_formats() in C:\xampplite\htdocs\transit\sites\all\modules\date\date\date_admin.inc on line 79

when trying to edit those fields. If the content type is called 'event' and the Date field that used to use the popup widget is called 'field_time' then this fatal error appears on the page admin/content/types/event/fields/field_time

The solution would be to default back to the ordinary text field when the popup module is disabled.