cannot delete quiz nodes, og groups, or users

Problem/Motivation

Currently when I try to delete a quiz node, I get Fatal error: Class name must be a valid object or a string in /sites/all/modules/contrib/quiz/question_types/quiz_question/quiz_question.module on line 652

Here is line 652:

$to_return = new $constructor($result_id, $question, $answer);

When I output the values, $result_id is the only variable with a value.

I have the same error when I try to delete a group or a user. Any assistance would be appreciated.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes