In my recent upgrade from MVC 3 to MVC 5, I encountered another issue/bug in system which JSONResult return null is working properly in MVC3 became an error in MVC5.

So, the code below,

return Json(null, JsonRequestBehavior.AllowGet);

should be changed to,

return Json(string.Empty, JsonRequestBehavior.AllowGet);

It should return empty string to avoid being recognized as error.

Cheers!