In this ticket (#419) you said to run bugsack/buggrabber so here's the info from that. Replicated this every day for weeks now on a level 120 Horde Retribution Paladin. This did not occur when I ran it on a level 120 Horde Windwalker Monk. It does not occur on Mythic Seat of the Triumvirate Viceroy Neshar.

10x BigWigs_Core\BossPrototype.lua:939: attempt to call field '?' (a nil value)
BigWigs_Core\BossPrototype.lua:939: in function `?'
BigWigs_Core\Core.lua:59: in function <BigWigs_Core\Core.lua:54>

Locals:
InCombatSkipped

22x BigWigs_Core\BossPrototype.lua:926: attempt to call field '?' (a nil value)
BigWigs_Core\BossPrototype.lua:926: in function `?'
BigWigs_Core\Core.lua:59: in function <BigWigs_Core\Core.lua:54>

Locals:
InCombatSkipped

21x BigWigs_Core\BossPrototype.lua:919: attempt to call field '?' (a nil value)
BigWigs_Core\BossPrototype.lua:919: in function `?'
BigWigs_Core\Core.lua:59: in function <BigWigs_Core\Core.lua:54>

Locals:
InCombatSkipped

Thanks. This has nothing do with LittleWigs and is actually a core issue.
It has been fixed in BigWigsMods/BigWigs@f453bc7 and will be in the next release.