In case you have not heard, Google released an update to the Nik Collection today.
https://support.google.com/nikcollection/answer/4400822?hl=en&ref_topic=3001406
This update brings more stability and improvements. The update should install automatically when your computer is online and Photoshop, Bridge, Lightroom and Aperture are closed.
Thanks for the update Stuart!
No mention of it correcting the issue with HDR Efex Pro 2 and inability to save edited image back to Aperture automatically. I will have to test this and see if it is fixed.