?:0: attempt to index a nil value

function animateMonster()

    print "*******     ZAAAAAAPPP!!!!!!!!!!!!!     *******"
        local function finishedAttackAnimation(self, event)
            if event.phase == "end" then
            group.gfx.elements.Npc8:removeEventListener('sprite', group.gfx.elements.Npc8)  -- This line causes error below.
            group.gfx.elements.Npc8:play ((currentMap.monsterList[M].npcName .. " Stance 2")  )
            currentMap.monsterList[M].npcTurnState = "moved"
            -- print (M, currentMap.monsterList[M].npcTurnState)
            npcActionTakingPlace = false
            print("DMHM-dispatch from 'attack-animate'");Runtime:dispatchEvent{name='dungeonMasterHasMoved'}

            end
        end
        group.gfx.elements.Npc8.sprite = finishedAttackAnimation
        group.gfx.elements.Npc8:addEventListener('sprite', group.gfx.elements.Npc8)
        group.gfx.elements.Npc8:play ((currentMap.monsterList[M].npcName .. " Attack 2") )

end

--[[
** This error only occurs when the animation has been triggered at least once, the game is reset, and the player encounters it again.  If the level is completely reloaded from IN-Game or if I attack and force him to do a hit animation, it goes away.

Runtime error
    ?:0: attempt to index a nil value
stack traceback:
    [C]: ?
    ?: in function '?'
    ?: in function <?:215>
]]