Recurring issue: Fatal error: Class 'EntityReference_SelectionHandler_Generic' not found

Hi there,

When defining a new handler I'm getting the error:
Fatal error: Class 'EntityReference_SelectionHandler_Generic' not found

But the that's coming from OG and not from the handler I created. I saw several ways to solve this one but none of them really helped.

I'm running on 7.x-1.1 but I can't update the module since the project is pretty complex. The right for me is to post a patch and hope other people will benefit from the patch.