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