Contextual filter "Display a summary" causes error and broken UI

Notice: Undefined property: stdClass::$unknown in draggableviews_handler_field_draggable->views_form() (line 137 of /sites/all/modules/draggableviews/views/draggableviews_handler_field_draggable.inc).

This notice is thrown, and an erroneous "Save" button appears on a view summary when the view is displayed without a contextual filter argument, and the contextual filter is set to "Display a summary" when the argument is not present.

A quick fix would be to simply not add the Save button and not execute the draggableviews_handler_field_draggable->views_form() when the view has been set to a "summary mode" (I made that term up) by the contextual filter setting.