You don't lose root if you uninstall the manager.
The Magisk core / Daemon is what you flash, and from your screenshot that appears to be on 25200 (stable)
Without logs I can't speak of your JS error, but PF does not use JS, it is written in Python, so I'm not sure what you're experiencing.
Just uninstall Magisk Manager, Install the stable and reboot.
Then give me support.zip if you are still having an issue.
to do with PixelFlasher. It appeared when I was trying to manually install the 25206 .apk I had downloaded from the github/magisk site. (Mentioned here earlier as the newest one without issues.)
My confusion over "uninstalling" the Magisk Manager stems from the fact that there's a button inside the app that says "Uninstall Magisk". Tapping that get's you a warning message of dire consequences if you proceed.
If you just uninstall it from the App Info screen, everything is fine--as you suggested.
I was able to install the 25200 manager, and it looks like PF will proceed properly.
 I just realized I did NOT flash to the inactive slot in PF. Will that be a problem?