Okay, figured it out if no-one else has yet
That message comes up when you type the command :
/nui bagbar ???
Where ??? should be one of the following :
on, off, mouseover
That message is a result of tests being made to ensure the correct information is being entered. Once this option is added to the GUI optional addon (if not already) that message shouldn't occur, assuming you have it installed.
At the same time bag scaling seems to have a similar problem. /nui bagscale 0 causes another error message to appear. Again, having the GUI option installed with stop this from happening too.