These LUA errors happen with both AllTheThings and MoveAnything installed, and persist with only these two addons installed, even after removing and reinstalling them. They do not occur when first logging into the game, but are consistent anytime /reload is used or characters are switched afterward. Here are the errors:

Message: Deferred XML Node object named Lib_UIDropDownMenuButtonTemplate already exists
Time: Sat Oct 13 16:07:43 2018
Count: 5

Message: Deferred XML Node object named Lib_UIDropDownListTemplate already exists
Time: Sat Oct 13 16:07:43 2018
Count: 5

Message: Deferred XML Node object named Lib_UIDropDownMenuTemplate already exists
Time: Sat Oct 13 16:07:43 2018
Count: 5

This is because both addons use the same Library and I believe it's MoveAnything that hasn't declared. TSM did the same thing until they declared it. Essentially it doesn't cause any problems other than an annoying box. Prior to BfA it use to be something that was hidden, but in BfA Blizzard decided to make it so you can see it.

Ah, I see. I also went to say the same thing on MoveAnything's website, and apparently the addon is basically abandoned with only minimal updating for BfA. So it seems the problem is using MoveAnything. Case closed, thank you. :)

Yeah, for now it's perfectly fine and won't cause you any issues other than the annoying popup. Which if you want you can get Buggrabber / Bugsacks and it puts them all in a hidden window.

I know at some point we are going to rewrite the dropdown menu to avoid an external library, just haven't had a chance to do it.

MoveAnything has other irritations that I was just dealing with. I think I'll just find an alternative for the few specific things I was using it for. Thank you for the suggestions!