56x TooltipItemIcon/TooltipItemIcon.lua:437: attempt to call method 'GetItem' (a nil value)
[string "@TooltipItemIcon/TooltipItemIcon.lua"]:437: in function <TooltipItemIcon/TooltipItemIcon.lua:429>
[string "=(tail call)"]: ?
[string "=[C]"]: in function securecallfunction' [string "@SharedXML/Tooltip/TooltipDataHandler.lua"]:60: in function <SharedXML/Tooltip/TooltipDataHandler.lua:55> [string "@SharedXML/Tooltip/TooltipDataHandler.lua"]:80: in function <SharedXML/Tooltip/TooltipDataHandler.lua:76> [string "@SharedXML/Tooltip/TooltipDataHandler.lua"]:116: in function <SharedXML/Tooltip/TooltipDataHandler.lua:106> [string "=[C]"]: in function SetAttribute'
[string "@SharedXML/Tooltip/TooltipDataHandler.lua"]:141: in function <SharedXML/Tooltip/TooltipDataHandler.lua:134>
[string "@SharedXML/Tooltip/TooltipDataHandler.lua"]:282: in function <SharedXML/Tooltip/TooltipDataHandler.lua:240>
[string "=[C]"]: in function securecallfunction' [string "@SharedXML/Tooltip/TooltipDataHandler.lua"]:237: in function ProcessInfo'
[string "@SharedXML/Tooltip/TooltipComparisonManager.lua"]:249: in function SetItemTooltip' ... [string "@SharedXML/Tooltip/TooltipDataHandler.lua"]:71: in function <SharedXML/Tooltip/TooltipDataHandler.lua:67> [string "@SharedXML/Tooltip/TooltipDataHandler.lua"]:135: in function <SharedXML/Tooltip/TooltipDataHandler.lua:134> [string "@SharedXML/Tooltip/TooltipDataHandler.lua"]:282: in function <SharedXML/Tooltip/TooltipDataHandler.lua:240> [string "=[C]"]: in function securecallfunction'
[string "@SharedXML/Tooltip/TooltipDataHandler.lua"]:237: in function <SharedXML/Tooltip/TooltipDataHandler.lua:236>
[string "=(tail call)"]: ?
[string "=[C]"]: ?
[string "=[C]"]: in function SetBagItem' [string "@FrameXML/ContainerFrame.lua"]:1504: in function UpdateTooltip'
[string "@FrameXML/GameTooltip.lua"]:498: in function <FrameXML/GameTooltip.lua:491>

Locals:
frame = ShoppingTooltip1 {
0 =
SetQuestLogRewardSpell = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetQuestPartyProgress = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
ProcessLineData = defined @SharedXML/Tooltip/TooltipDataHandler.lua:303
SetPvpTalent = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetExistingSocketGem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetHyperlink = defined =[C]:-1
SetUnit = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetAzeriteEssence = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetVoidDepositItem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetVoidWithdrawalItem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetInfoBackdropStyle = defined @SharedXML/Tooltip/TooltipDataHandler.lua:338
SetSocketGem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetLootCurrency = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetSlottedKeystone = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetItemInteractionItem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetLootItem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetOwnedItemByID = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetQuestLogSpecialItem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetRecipeReagentItem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetBuybackItem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetWeeklyReward = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetMerchantItem = defined =[C]:-1
SetPvpBrawl = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
BottomOverlay = Texture {
}
SetSpellByID = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetSocketedRelic = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
ProcessInfo = defined @SharedXML/Tooltip/TooltipDataHandler.lua:236
ProcessLines = defined @SharedXML/Tooltip/TooltipDataHandler.lua:293
NineSlice = Frame {
}
SetInventoryItem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
info =

The GetItem method is missing for tooltips that did not inherit GameTooltipTemplate (in this case ShoppingTooltip1)

Checked 10.0.2 code to see how they implemented GetItem in GameTooltipTemplate

Note: still need to use GetItem in Classic clients

Above commit appears to have fixed this issue.