Started happening after I reset my profile and had put my hunter's usual buttons back on the bar, I now get this every time I log in on that toon.

14x Neuron\Neuron-Buttons.lua:1345: attempt to index a nil value
Neuron\Neuron-Buttons.lua:1345: in function MACRO_UpdateUsableSpell' Neuron\Neuron-Buttons.lua:1453: in function MACRO_UpdateButton'
Neuron\Neuron-Buttons.lua:1328: in function MACRO_UpdateAll' Neuron\Neuron-Buttons.lua:1661: in function ?'
Neuron\Neuron-Buttons.lua:1757: in function `MACRO_OnEvent'
Neuron\Neuron-Buttons.lua:3377: in function <Neuron\Neuron-Buttons.lua:3377>

Locals:
InCombatSkipped

Yeah you're not the only person getting this on their hunters. Is it only your hunter?

I tried to replicate this on my hunter and I just can't

I eventually realized that when I first dragged the Command Pet (Primal Rage in this case) spell out of my spell book to the action bar it actually put it in as /cast Primal Rage(), that's what was causing the problem (I duplicated by just manually creating the button with Primal Rage and relogged, that brought the error back). At some point while playing earlier I noticed that the button wasn't working so I removed it and re-created it, that time it did create it correctly as /cast Command Pet() and the problem went away at that point.

So, the issue could be when a pet spell is put on a non-pet action bar button or related to the Command Pet replaceable spell (not sure how else to describe that).

To quickly test that I tried creating a button with just /cast Growl() at which point I got non-stop LUA errors:

258x integer overflow attempting to store 3238005337
[C]: in function GetSpellInfo' Neuron\Neuron-Buttons.lua:1345: in function MACRO_UpdateUsableSpell'
Neuron\Neuron-Buttons.lua:1453: in function MACRO_UpdateButton' Neuron\Neuron-Buttons.lua:1490: in function MACRO_OnUpdate'
Neuron\Neuron-Buttons.lua:3391: in function <Neuron\Neuron-Buttons.lua:3391>

Locals:
(*temporary) = 3238005337

So, probably related to pet spells in general. Hopefully that helps and let me know if you need anything else.

Tried on my warlock, put the demon dps ability on the action bar and it worked immediately but after relogging I got another bunch of integer errors but not the original one reported above. Not sure what to conclude from that but hopefully gives you enough info to test/duplicate. This integer issue should probably be reported separately, let me know if you need that done.

Message: integer overflow attempting to store 3238006404
Time: Sat Aug 4 18:42:09 2018
Count: 1
Stack: integer overflow attempting to store 3238006404
Interface\AddOns\Neuron\Neuron-Buttons.lua:1328: in function MACRO_UpdateAll' Interface\AddOns\Neuron\Neuron-Buttons.lua:1661: in function ?'
Interface\AddOns\Neuron\Neuron-Buttons.lua:1757: in function `MACRO_OnEvent'
Interface\AddOns\Neuron\Neuron-Buttons.lua:3385: in function <Interface\AddOns\Neuron\Neuron-Buttons.lua:3385>

Locals: self =

fixed, thanks @veyh !