How do I use a ModuleScript to save player data?

So I'm writing an inventory ModuleScript that saves a table to the player's UserID through a datastore, to do this, I've loosely followed the Roblox Developer Hub tutorial.

local DS = game:GetService("DataStoreService")      
local Store = DS:GetDataStore("Datastore")          
local AUTOSAVE_INTERVAL = 10                        
local SessionData = {}  
local PlayerInventoryManager = {}

function PlayerInventoryManager:ChangeInventory(player, item, value)
    local UserID = "Player_" .. player.UserId
    assert(typeof(SessionData[UserID][item]) == typeof(value), "ChangeInventory error: data types do not match")
    if typeof(SessionData[UserID][item]) == "number" then
        SessionData[UserID][item] = SessionData[UserID][item] + value
    else
        SessionData[UserID][item] = value
    end
end

local function SetupPlayerData(player)              
    local UserID = "Player_" .. player.UserId       
    local Data = Store:GetAsync(UserID)             
    if Data then                                    
        SessionData = Data
        for i, v in next, SessionData do
            print(i, v)
        end
    else 
        SessionData[UserID] = {Stone = 0}
        for i, v in next, SessionData do
            print(i, v)
        end
        warn("No data found for " .. UserID)        
    end
end

local function savePlayerData(UserID)               
    if SessionData[UserID] then                     
        Store:SetAsync(UserID, SessionData[UserID])
    end
end

local function saveOnExit(player)                  
    local UserID = "Player_" .. player.UserId   
    savePlayerData(UserID)
end

local function autoSave()                           
    while wait(AUTOSAVE_INTERVAL) do                
        for UserID, data in pairs(SessionData) do   
            savePlayerData(UserID)                  
        end
    end
end

spawn(autoSave)                                     
game.Players.PlayerAdded:Connect(SetupPlayerData)   
game.Players.PlayerRemoving:Connect(saveOnExit) 

return PlayerInventoryManager

I want scripts to be able to access SessionData so it can add and remove data before being copied to PlayerInventoryManager and saved, so I wrote this Script in ServerScriptService:

game.Players.PlayerAdded:Connect(function(player)
    local module = require(script.Parent.ModuleScript)
    local item = "testsss"
    local value = 1
    wait(1)
    module:ChangeInventory(player, item, value)
end)

However after this, I get the error ServerScriptService.ModuleScript:9: attempt to index nil with 'testsss'.

Does anyone know how to fix this, and/or point me in the direction of how to properly use ModuleScripts for saving player data?