Attempted to index nil with 'FindFirstChild' in a module script?

I am working on a anticheat that makes it so no type of exploits are possible. Even Btools! I seem to be getting an error in my module that says attempted to index nil with 'FindFirstChild' on line 29. Any help would be greatly appreciated.

local module = {}
ValidKeys = {}
GUiNumber = {}
game.Players.PlayerAdded:Connect(function(plr)
player = plr
end)
ValidGui1 = game.StarterGui.ValidGui
function module.Store(key, UiNumber)
	table.insert(ValidKeys, key)
	table.insert(GUiNumber, UiNumber)
end
function module.GetFullPath(Ui)
    partPath = Ui:GetFullName()
	local appendStr = "game."
end
function module.GiveGuiKey(Ui)
	local key = Instance.new("StringValue")
	local hasgivenkey = nil
	module.GetFullPath(Ui)
	key.Parent = Ui
	key.Name = "key"
	key.Value = math.random(154122412, 521532125)
		hasgivenkey = true
	module.Store(key.Value, partPath)
end
function module.CheckForExploits()
	for i,v in pairs(ValidKeys) do
		local guinumber = GUiNumber[v]
		local iskey = guinumber:FindFirstChild("key")
		if iskey then
			local key = GUiNumber[v]:FindFirstChild("key")
			if key.Value ~= v then
				script.RemoteEvent:FireServer(plr)
			end
		else
			script.RemoteEvent:FireServer(plr)
		end
	end
end
function module.DistributeKeys(get)
	for i, v in pairs(get:GetChildren()) do
		if v:IsA("ScreenGui") and not v:FindFirstChild("key") then
			module.GiveGuiKey(v)
		end
	end
	print(GUiNumber)
end
return module