Bagsync may still be attempting to call SetBackdrop. This only happens when the legacy addon "BadPet" is enabled AND you mouse over a battle pet. Just thought you should know.

207x ...faceBagSync\libs\LibQTip-1.0\LibQTip-1.0-47.lua:540: attempt to call method 'SetBackdrop' (a nil value)
[string "@BagSync\libs\LibQTip-1.0\LibQTip-1.0-47.lua"]:540: in function <...faceBagSync\libs\LibQTip-1.0\LibQTip-1.0.lua:534>
[string "@BagSync\libs\LibQTip-1.0\LibQTip-1.0-47.lua"]:127: in function Acquire' [string "@BagSync\wireframe\tooltip.lua"]:328: in function TallyUnits'
[string "@BagSync\wireframe\tooltip.lua"]:663: in function <BagSync\wireframe\tooltip.lua:658>
[string "=[C]"]: in function FloatingBattlePet_Show' [string "@FrameXML\FloatingPetBattleTooltip.lua"]:57: in function FloatingBattlePet_Toggle'
[string "@FrameXML\ItemRef.lua"]:197: in function <FrameXML\ItemRef.lua:11>
[string "=[C]"]: ?
[string "=[C]"]: ?
[string "@Blizzard_CombatLog\Blizzard_CombatLog.lua"]:3520: in function `SetItemRef'
[string "@FrameXML\ChatFrame.lua"]:3906: in function <FrameXML\ChatFrame.lua:3905>
[string "=(tail call)"]: ?
[string "=(tail call)"]: ?
[string ":OnHyperlinkClick"]:1: in function <[string ":OnHyperlinkClick"]:1>
[string "=[C]"]: ?

Locals:
tooltip = {
0 =
scrollChild = {
}
scrollFrame = {
}
}
key = "FloatingBattlePetTooltip"
backdrop =

{
backdropColor = {
}
edgeSize = 16
tileEdge = true
backdropBorderColor = {
}
tileSize = 16
edgeFile = "Interface/Tooltips/UI-Tooltip-Border"
tile = true
bgFile = "Interface/Tooltips/UI-Tooltip-Background"
insets = {
}
}
(*temporary) = nil
(*temporary) = {
0 =
scrollChild = {
}
scrollFrame = {
}
}
(*temporary) = {
backdropColor = {
}
edgeSize = 16
tileEdge = true
backdropBorderColor = {
}
tileSize = 16
edgeFile = "Interface/Tooltips/UI-Tooltip-Border"
tile = true
bgFile = "Interface/Tooltips/UI-Tooltip-Background"
insets = {
}
}
(*temporary) = "attempt to call method 'SetBackdrop' (a nil value)"
GameTooltip = GameTooltip {
0 =
OnBackdropLoaded = defined @SharedXML\Backdrop.lua:205
ItemTooltip = {
}
SetHyperlink = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wrapper.lua:96
RightEdge = {
}
SetVoidWithdrawalItem = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wrapper.lua:96
ClearBackdrop = defined @SharedXML\Backdrop.lua:335
SetLootItem = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wrapper.lua:96
numMoneyFrames = 1
SetRecipeReagentItem = defined =[C]:-1
SetBuybackItem = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wrapper.lua:96
SetMerchantItem = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wrapper.lua:96
SetupTextureCoordinates = defined @SharedXML\Backdrop.lua:267
TopRightCorner = {
}
GetEdgeSize = defined @SharedXML\Backdrop.lua:241
updateTooltip = 0.184000
SetInventoryItem = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wrapper.lua:96
TopEdge = {
}
SetLootRollItem = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wrapper.lua:96
SetAuctionSellItem = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wrapper.lua:96
SetBagItem = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wrapper.lua:96
BottomRightCorner = {
}
SetBackdropColor = defined @SharedXML\Backdrop.lua:452
LeftEdge = {
}
SetAuctionItem = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wrapper.lua:96
SetCurrencyByID = defined =[C]:-1
textLeft1Font = "GameTooltipHeaderText"
SetCurrencyToken = defined =[C]:-1
SetGuildBankItem = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wrapper.lua:96
SetVoidItem = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wrapper.lua:96
SetBackdropBorderColor = defined @SharedXML\Backdrop.lua:475
TextRight1 = GameTooltipTextRight1 {
}
ApplyBackdrop = defined @SharedXML\Backdrop.lua:347
SetBackdrop = defined @SharedXML\Backdrop.lua:382
TopOverlay = {
}
GetBackdropBorderColor = defined @SharedXML\Backdrop.lua:462
GetBackdrop = defined @SharedXML\Backdrop.lua:400
SetUnitDebuff = defined =[C]:-1
comparing = false
__tooltipUpdated = false
SetQuestItem = defined =[C]:-1
hasMoney = 1
SetBorderBlendMode = defined @SharedXML\Backdrop.lua:319
SetBackpackToken = defined =[C]:-1
SetCurrencyTokenByID = defined =[C]:-1
BottomLeftCorner = {
}
SetInboxItem = defined @TradeSkillMaster\LibTSM\Service\ItemTooltipClasses\Wr

It appears that the LibQTip is being sent old tooltips that don't have the new Backdrop Mixin. So therefore, it's missing the necessary functions for it to properly display. This is what fires off the error. I'm pretty sure it's the legacy addon BadPet, that is causing this problem. Regardless, I've filed a ticket with the creator of LibQTip.

Please see the following issue.
https://www.wowace.com/projects/libqtip-1-0/issues/48