Error with Aggregate devices on OS X

Hi John,
I do have REW V5,19 Beta 8 installed on OSX Sierra. I have created an "Aggregate" device with multiple outputs as I need to output simultaneously audio from my Focusrite audio card and Bluetooth device, for testing purpose.
It seems that REW doesn't send audio through Aggregate devices, thought my Smart software is able to do it, is something relate with Java? Is there a way to overcome this?
When I try to start the REW generator with the Aggregate device selected I get the error in the picture attached.
Thank you.
Oliver

Hi John,
thanks to reply.
I do have also an aggregate device with only two Focusrite usb interfaces, and even with that doesn't work. the aggregate device is showed under the output menu in REW but when I play the generator i get the same error.