fix: Combobox ConVar change callback deletion

Fixes #1068

We need to remove the callback in a timer, because otherwise the ConVar change callback code
will throw an error while looping over the callbacks.
This happens, because the callback is removed from the same table that is iterated over.
Thus, the table size changes while iterating over it and leads to a nil callback as the last entry.

See https://github.com/Facepunch/garrysmod/blob/1b512930d1f8fb1acf6235e584c4ec1ff84e9362/garrysmod/lua/includes/modules/cvars.lua#L44
and
https://github.com/Facepunch/garrysmod/blob/1b512930d1f8fb1acf6235e584c4ec1ff84e9362/garrysmod/lua/includes/modules/cvars.lua#L97

here the code uses table.Remove, which causes the table to be reindexed and resized, this is the root cause for the problem.
We should thus avoid removing callbacks in the callback itself.

gamemodes/terrortown/gamemode/client/cl_vskin/vgui/dcombobox_ttt2.lua

@@ -338,23 +338,29 @@ end
 
 local convarTracker = 0
 ---
--- @param Panel menu to set the value of
+-- @param Panel panel to set the value of
 -- @param string conVar name of the convar
-local function AddConVarChangeCallback(menu, conVar)
-	convarTracker = convarTracker % 1023 + 1
-	local myIdentifierString = "TTT2F1MenuConVarChangeCallback" .. tostring(convarTracker)
-
-	local function OnConVarChangeCallback(conVarName, oldValue, newValue)
-		if not IsValid(menu) then
-			cvars.RemoveChangeCallback(conVarName, myIdentifierString)
+local function AddConVarChangeCallback(panel, conVar)
+	convarTracker = convarTracker + 1
+	local myIdentifierString = "TTT2F1MenuComboboxConVarChangeCallback" .. tostring(convarTracker)
+
+	local callback = function(conVarName, oldValue, newValue)
+		if not IsValid(panel) then
+			-- We need to remove the callback in a timer, because otherwise the ConVar change callback code
+			-- will throw an error while looping over the callbacks.
+			-- This happens, because the callback is removed from the same table that is iterated over.
+			-- Thus, the table size changes while iterating over it and leads to a nil callback as the last entry.
+			timer.Simple(0, function()
+				cvars.RemoveChangeCallback(conVarName, myIdentifierString)
+			end)
 
 			return
 		end
 
-		menu:SetValue(newValue, true)
+		panel:SetValue(newValue, true)
 	end
 
-	cvars.AddChangeCallback(conVar, OnConVarChangeCallback, myIdentifierString)
+	cvars.AddChangeCallback(conVar, callback, myIdentifierString)
 end
 
 ---