I was doing nothing special, i was walking in Zereth Moris and this error show up.
I hoppe this will help you to make your addon even better.

1x MythicPlusTimer/progress.lua:181: attempt to call method 'GetUnit' (a nil value)
[string "@MythicPlusTimer/progress.lua"]:181: in function <MythicPlusTimer/progress.lua:175>
[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 <SharedXML/Tooltip/TooltipDataHandler.lua:236>
[string "=(tail call)"]: ?
[string "@HandyNotes_ShadowlandsTreasures/handler/handler.lua"]:908: in function <.../HandyNotes_ShadowlandsTreasures/handler/handler.lua:743>
[string "=(tail call)"]: ?
[string "@HandyNotes_ShadowlandsTreasures/handler/handler.lua"]:949: in function <.../HandyNotes_ShadowlandsTreasures/handler/handler.lua:925>
[string "=[C]"]: ?
[string "@HandyNotes/HandyNotes.lua"]:56: in function <HandyNotes/HandyNotes.lua:51>
[string "@HandyNotes/HandyNotes.lua"]:166: in function <HandyNotes/HandyNotes.lua:165>

Locals:
tooltip = HandyNotes_ShadowlandsTreasuresComparisonTooltip {
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 @SharedXML/Tooltip/TooltipDataHandler.lua:444
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
AllTheThingsOnTooltipClearedHook = true
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
SetLFGDungeonShortageReward = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetOwnedItemByID = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
InternalProcessInfo = defined @SharedXML/Tooltip/TooltipDataHandler.lua:240
SetQuestLogSpecialItem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetRecipeReagentItem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetBuybackItem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
hasMoney = 1
SetWeeklyReward = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetMerchantItem = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
SetPvpBrawl = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
BottomOverlay = Texture {
}
SetSpellByID = defined @SharedXML/Tooltip/TooltipDataHandler.lua:444
numMoneyFrames = 1
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 =

Tooltip issues should be fixed in 5.0.2