Lightroom CC Slideshow error

Hi. It's been a while now that in Lightroom CC (now Lightroom Classic!) when I finish editing images and move on to the Slideshow module that, after the Preview has completed, and the Slideshow should commence, I have nothing but a black screen with some white borders. The slideshow is set to music, so all that happens is that the music commences, there is a black screen etc but no active slideshow. Images then begin way down the line......! I'm on a 3 year old Mac Pro Retina.

Until, I close Lightroom and re-open it again. Then, behold, the Slideshow module actually does its job!

So, can any of you gurus enlighten me? I've tried deselecting 'Graphics Processor' from Preferences to no avail. I would have thought that this latest update in LR 'Classic' would have all been history. Quite unprofessional when a client is there waiting to see the Slideshow, as you can imagine.

Your experienced opinions, please.

Many thanks.