Attempting to edit self-enrollment instance limited to a cohort that has been deleted produces generalexceptionmessage error

To reproduce:

1. Create cohort at category level
2. In a course within that category, add a Self-Enrollment instance, and set "Only cohort members" to the new cohort created in #1
3. Delete cohort created in #1 
4. Attempt to edit Self-Enrollment instance created in #2. This will produce the following error:  Exception - Call to a member function get_record() on null

Full error reporting (with debugging on):

{{Debug info: }}
Error code: generalexceptionmessage
Stack trace:
  - line 779 of /enrol/self/lib.php: Error thrown
  - line 52 of /enrol/editinstance_form.php: call to enrol_self_plugin->edit_instance_form()
  - line 204 of /lib/formslib.php: call to enrol_instance_edit_form->definition()
  - line 69 of /enrol/editinstance.php: call to moodleform->__construct()
Output buffer: <br /> <b>Notice</b>: Undefined variable: DB in <b>/var/www/html/enrol/self/lib.php</b> on line <b>779</b><br />