Module saying attempted to index nil with FindFirstChild(). Is there any way around this?

I am working on an anticheat for roblox that will make it so almost every hack will not be possible to preform in a game. However, the module script for it keeps returning attempting to index nil with FindFirstChild in the output. How would I go about fixing this? Here is the module. the error is on line 29. It should not be returning nil, as I have checked the path of the object and it is there. The path is game.Players.LocalPlayer.PlayerGui.ValidGui.ScreenGui. (Screen Gui being v.) I am getting the local player from a server script using the game.Players.PlayerAdded:Connect(function(plr). I then send the playerGui to the module using the Distributekeys() function. Server script is after the module script.

local module = {}
ValidKeys = {}
GUiNumber = {}
-- game.Players.PlayerAdded:Connect(function(plr) don't mind this function, I am not using it.
-- 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

server script looks something like this.

module = require(game:GetService("ReplicatedStorage").Module)
game.Players.PlayerAdded:Connect(function(plr)
module.DistributeKeys(plr.PlayerGui)
while wait() do
module.CheckForExploits()
game:GetService("ReplicatedStorage").module.RemoteEvent.OnServerEvent:Connect(
function(plr) --  continuation of line 6.
plr:Kick("We have detected a use of exploits. Roblox does not support exploits.")
end)
end
end)