Readout:

24x CraftSim/Util/Tooltip.lua:13: attempt to call method 'GetItem' (a nil value)
[string "@CraftSim/Util/Tooltip.lua"]:13: in function <CraftSim/Util/Tooltip.lua:12>
[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 "@FrameXML/GameTooltip.lua"]:527: in function GameTooltip_ShowCompareItem'
[string "@SharedXML/Tooltip/TooltipDataRules.lua"]:161: in function func' [string "@SharedXML/Tooltip/TooltipDataHandler.lua"]:49: in function <SharedXML/Tooltip/TooltipDataHandler.lua:44> [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 ProcessInfo' [string "@Blizzard_EncounterJournal/Blizzard_EncounterJournal.lua"]:2092: in function EncounterJournal_SetTooltipWithCompare'
[string "*Blizzard_EncounterJournal.xml:1382_OnEnter"]:3: in function <[string "*Blizzard_EncounterJournal.xml:1382_OnEnter"]:1>

I put a check for this in the next release :)
2.0+