I think this change is required to improve the developer experience when working on IDEA (to find resources more easily).
But I'm not sure that this is the best way.
The behaviour here has changed from the original version. Before getConfigResources== null was valid, now it throws an exception.
The new method getMultipleConfigResources has a bad name. I would use getConfigResourceArray or similar.
Original method getConfigResources has to be deprecated. 
In the original version it was abstract, maybe it has to thrown an exception when its called from the base class.
I would invoke getConfigResources directly from getMultipleConfigResources.
It would be good to have a separate commit for this (I can help)