Authors CurseForge

Well, you're creating a lot of unnecessary and generically-named global variables, so it's not outside the realm of possibility that your addon's variables are being unexpectedly overwritten by other addons, or even the default UI, if they are leaking globals by the same name. As a general rule, every variable you define should be local unless there is a specific need for it to be global (eg. saved variables must be global).

Anyway, I guess your error is coming from these lines:

	 _,link = GameTooltip:GetItem()
	id = tonumber(link:match('item:(%d+)'))

...in which case it would indicate that the GameTooltip is not displaying an item when this function is called. This can happen if the item it's been asked to display isn't cached yet, and it has to retrieve item info from the server. You should add a check to make sure the link variable has a value before proceeding with the rest of your function:

	 _,link = GameTooltip:GetItem()
	[b][color="#007f4f"]if not link then return end[/color][/b]
	id = tonumber(link:match('item:(%d+)'))

There are aslo a couple other (mostly minor, aside from the many globals) issues with your code:

rh_RawMats = {}

[b][color="#007f4f"]local[/color][/b] lineAdded = false

local function RH_MatsShow(tooltip, ...)
	if IsShiftKeyDown() then
		[b][color="#007f4f"]local[/color][/b] __,link = GameTooltip:GetItem()
		[b][color="#007f4f"]local[/color][/b] id = tonumber(link:match('item:(%d+)'))
		[b][color="#007f4f"]local[/color][/b] itemid = rh_RawMats[id]
		if itemid and not lineAdded then
			GameTooltip:AddLine("" ..itemid,0,0,0,0)
			[COLOR="#007f4f"][I]-- ^ There's no need to concatenate the itemid with a blank string,
			-- and you are currently making this text black, which is nearly
			-- invisible since the tooltip background is also black.[/I][/COLOR]
			lineAdded = true
		end
	end
	return rh_RawMats
	[COLOR="#007f4f"][I]-- ^ There's not really any point in returning this value, since this
	-- function is set as a script, whose return value is not captured or
	-- used by anything. This value is also already available as a global
	-- variable, so there's no need to call a function to get it if you
	-- want to use it somewhere else. I'd just delete this line.[/I][/COLOR]
end

local function RH_MatsHide(tooltip, ...)
	lineAdded = false
end

GameTooltip:HookScript("OnTooltipSetItem", RH_MatsShow)
GameTooltip:HookScript("OnTooltipCleared", RH_MatsHide)

--mat proffession tooltip
rh_Whatreagent = {}

lineAdded = false
[COLOR="#007f4f"][I]^ -- You already defined this variable on line 3, so you do not need to
-- define it again. If this code is actually in a different file, make
-- this variable local like the one on line 3. If it's in the same file
-- either remove this line or -- since this variable appears to serve
-- a different purpose -- make it local and give it a different name.[/I][/COLOR]

local function RH_ReagentShow(tooltip, ...)
	[b][color="#007f4f"]local[/color][/b] _, link = GameTooltip:GetItem()
	[b][color="#007f4f"]local[/color][/b] id = tonumber(link:match('item:(%d+)'))
	[b][color="#007f4f"]local[/color][/b] myreagent = rh_Whatreagent[id];
	if myreagent and not lineAdded then
		GameTooltip:AddLine("|cff00bfff\nReagent used for:|cfff0e68c"..myreagent,0,0,0,0)
		[COLOR="#007f4f"][I]-- ^ Same problem as the mats line. Stop making text black! This
		-- text contains its own color codes, so it's not as bad, but in
		-- this case there's no point in passing color values at all.
		-- Also, don't prefix your added line with a line break, or use
		-- line breaks in the middle of your text. If you want to add
		-- multiple lines, call AddLine multiple times. If you want to
		-- add a blank line, call AddLine with a " " space as the text.[/I][/COLOR]
		lineAdded = true
	end
end

local function RH_ReagentHide(tooltip, ...)
	lineAdded = false
end

GameTooltip:HookScript("OnTooltipSetItem", RH_ReagentShow)
GameTooltip:HookScript("OnTooltipCleared", RH_ReagentHide)