Notice: Undefined index: entity type in views_handler_field_entity->init() (line 44 of .../views_handler_field_entity.inc).

When setting up views that make use of this specific handler I get the following error:
Notice: Undefined index: entity type in views_handler_field_entity->init() (line 44 of .../views_handler_field_entity.inc).

This error seems to be related to adding a "Quiz: Quiz Node NID" Relationship from another base table. #1866404: Add "Take quiz" link as Views field seems to be running into this issue when trying to test another patch.

See also #1714934: Notice: Undefined index: entity type in views_handler_field_entity->init() (line 44 of .../views_handler_field_entity.inc). and #1950762: Breaks when adding "sign-up" link to view for other tickets discussing this issue.

The solution seems to be pretty simple. We just need to add the "entity type" value to the Views Data the Quiz module exposes in hook_views_data(). I have attached a patch that does this, and to got rid of the error message for me.