Which version of MoveAnything are you using ?
It appears to be up to date ( Jan 2020 ) on Curse which should have got rid of any errors like this when it was first released for the expansion.
https://www.curseforge.com/wow/addons/move-anything
However, I can see from the bug reports that the same error you are seeing has been reported there.
Worse case Scenario is that the changes introduced by Blizzard this expansion has made addons like MoveAnything not viable anymore. However, nUI has mover frames for most frames and I only came across one of these errors and no amount of research/code changing would resolve it, so I took that element of nUI out of the equation for the time being.
For your reference to when this error was first noticed by myself ...
https://www.wowinterface.com/forums/...ad.php?t=57209
It highlights different scenarios which triggers the error and other similar scenarios which don't.