Sidecar showing blacks screen

This is both a question and solution.

I searched for questions about this problem but couldn't find any, so once I finally figured out a solution I thought I would share it.

I use Sidecar with my iPadPro and Mac (MBP 15 inch, 2018). It only works some of the time. When not working, the iPad has the sidebar with the keyboard and return, option, control etc icons but the rest of the iPad screen is black. I can drag windows onto this black space but because they aren't shown I can't get them back. This just proves that the MacOS thinks the display is there, but it doesn't actually show anything.

FIX: Once the iPad is connected using Sidecar, go to the Airplay icon (blue square) in the top menu bar and select "Mirror Built-in Retina Display". This causes the iPad to actually show something (a mirror of the MacOS screen), then go to the Airplay icon again but select "Use As Separate Display". Hey presto it works. Kind of annoying that I have to do this more than half the time to get it working!!!!

And sometimes it just goes black again, so I do my little 'mirror', 'separate display' and it works again.