Visually the tooltips look fine but they seem to throw up .lua errors with ElvUI enabled.
There are no issues if ElvUI is disabled, but it's a popular UI addon so this could potentially affect a lot of users. An error fires off for every tooltip event, leading to the game recommending addons be disabled due to the volume of errors. It happens when you mouse over an NPC, generating a tooltip. The error fires off once upon the tooltip appearing and does not recur until you mouse over another NPC on the map.

Message: ...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1241: attempt to index field '?' (a nil value)
Time: Wed Jul 25 17:17:59 2018
Count: 3
Stack: ...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1241: attempt to index field '?' (a nil value)
(tail call): ?
...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1241: in function <...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1239>
(tail call): ?
...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1831: in function UpdatePullButtonNPCData' ...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1900: in function ReloadPullButtons'
...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1492: in function <...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1477>
...Ons\MethodDungeonTools\BattleForAzeroth\Freehold.lua:52: in function UpdateMap' ...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1518: in function UpdateToDungeon'
...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1438: in function <...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1431>
(tail call): ?
[C]: ?
[string "safecall Dispatcher[3]"]:9: in function <[string "safecall Dispatcher[3]"]:5>
(tail call): ?
...ace\AddOns\HandyNotes\Libs\AceGUI-3.0\AceGUI-3.0.lua:314: in function Fire' ...es\Libs\AceGUI-3.0\widgets\AceGUIWidget-DropDown.lua:442: in function <...es\Libs\AceGUI-3.0\widgets\AceGUIWidget-DropDown.lua:433> (tail call): ? [C]: ? [string "safecall Dispatcher[3]"]:9: in function <[string "safecall Dispatcher[3]"]:5> (tail call): ? ...ace\AddOns\HandyNotes\Libs\AceGUI-3.0\AceGUI-3.0.lua:314: in function Fire'
...s\AceGUI-3.0\widgets\AceGUIWidget-DropDown-Items.lua:351: in function <...s\AceGUI-3.0\widgets\AceGUIWidget-DropDown-Items.lua:341>

Locals:

and

Message: ...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:2474: attempt to index field 'Tooltip' (a nil value)
Time: Wed Jul 25 17:23:44 2018
Count: 1
Stack: ...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:2474: attempt to index field 'Tooltip' (a nil value)
[C]: ?
...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:2474: in function <...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:2470>
[C]: in function Show' ...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1079: in function ActivatePullTooltip'
...nTools\AceGUIWidget-MethodDungeonToolsPullButton.lua:46: in function <...nTools\AceGUIWidget-MethodDungeonToolsPullButton.lua:37>

Locals:

What version of the AddOn are you using?

Well spotted -- I was using 2.2.0, Foolishly forgot to check to see if a newer version is out. Now I am running 2.2.1

I am getting two .lua errors, again I can confirm it ONLY occurs in conjuction with ElvUI 10.77. No other addons cause this clash, not even my tooltip addon.

The first is when mousing over a mob on the map:

Message: ...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:2551: attempt to index field 'Tooltip' (a nil value)
Time: Wed Jul 25 22:02:39 2018
Count: 7
Stack: ...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:2551: attempt to index field 'Tooltip' (a nil value)
[C]: ?
...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:2551: in function <...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:2547>
[C]: in function Show' ...terface\AddOns\MethodDungeonTools\DungeonEnemies.lua:215: in function DisplayBlipTooltip'
...terface\AddOns\MethodDungeonTools\DungeonEnemies.lua:69: in function <...terface\AddOns\MethodDungeonTools\DungeonEnemies.lua:65>

Locals:

And the second is when mousing over a pull on the list to the right. I moused over "1" (the first pull number above + "Add pull") to generate this error:

Message: ...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:2551: attempt to index field 'Tooltip' (a nil value)
Time: Wed Jul 25 22:05:23 2018
Count: 7
Stack: ...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:2551: attempt to index field 'Tooltip' (a nil value)
[C]: ?
...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:2551: in function <...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:2547>
[C]: in function Show' ...ace\AddOns\MethodDungeonTools\MethodDungeonTools.lua:1094: in function ActivatePullTooltip'
...nTools\AceGUIWidget-MethodDungeonToolsPullButton.lua:46: in function <...nTools\AceGUIWidget-MethodDungeonToolsPullButton.lua:37>

Locals:

It appears you toggled off the Tooltip module in ElvUI. Weird decision by you but the bug will be fixed in the next version.

I disabled the tooltip skinning module because I use TipTac for my tooltips; Elv's customisation for tooltips is rather poor. Not that weird :D

Thank you for doing what you can to help. I appreciate it a lot!