I use the Google Nik plugins (v2, 64-Bit) now and then, but have given up trying from within Xara Photo & Graphic Designer (v16.1.1). The best way I find is to export your bitmap image and use the plugins externally. You can drag and drop image files straight onto the Nik plugin icon to open (including .png files, which is not an option from within the plugin menu).

Within Xara you can get way with using some of the Efex plugins, but I've never got Analog, Color, HDR and Silver to work at all.

If I were going to chance using any of the ones that appear to work (Define, Sharpener Pro, Viveza) my workflow would be:

- make a copy of what you want to filter (as a backup)
- apply plugin to this copy
- lock the Effects (click the padlock on the Fx toolbar)
- immediately make new bitmap copy of result
- delete filtered copy (to prevent crashing)
- save

One thing to note is that after applying one of these plugins, you are not returned to the Xara program screen.