Sidecar only shows a black screen on iPad until reboot

I found that if you go to display preferences -> arrangement -> tick and then untick mirror display option it will work but I would have to repeat this step every time I use sidecar :)

when you try to reconnect sidecar, you can click mirror display after connecting your iPad.After that, you will find your iPad get bright.Then canceling mirror operation.

Toggle mirror displays on and off by pressing command-F1 twice.

In case you have multiple displays connected (and one of them is iPad), toggling mirror off/on was not working for me. What helped was to disconnect other displays and then try toggling mirroring tricks when only the iPad is connected.