After some testing this is coming from somewhere in the SVUI_Core portion of the add-on

Message: ...erface\AddOns\Blizzard_InspectUI\InspectPVPFrame.lua:80: Usage: UnitFactionGroup("unit" [,checkDisplayRace])
Time: Mon Mar 18 13:56:29 2019
Count: 1
Stack: ...erface\AddOns\Blizzard_InspectUI\InspectPVPFrame.lua:80: Usage: UnitFactionGroup("unit" [,checkDisplayRace])
[C]: in function UnitFactionGroup'
...erface\AddOns\Blizzard_InspectUI\InspectPVPFrame.lua:80: in functionInspectPVPFrame_Update'
...erface\AddOns\Blizzard_InspectUI\InspectPVPFrame.lua:60: in function <...erface\AddOns\Blizzard_InspectUI\InspectPVPFrame.lua:58>

Locals: (*temporary) = nil