[?] Data Store Indexing Nil Value

I've been trying to update a value from a data store, what am I doing wrong?

Error: ServerScriptService.DataStore:11: attempt to index field '?' (a nil value)

Code initiating player data (Module Script):

local PlayerDataHandler_module = {}

local module_Useful = require(game:GetService("ReplicatedStorage").Scripts.ModuleScripts.module_Useful)
local DataStoreService = game:GetService("DataStoreService")
local PlayerData = DataStoreService:GetDataStore("PLAYER_DATA")
local Http = game:GetService("HttpService")

local SessionData = {}

local AUTOSAVE_INTERVAL = 120


function PlayerDataHandler_module:ReturnStat(Player, ItemName)
    return SessionData[Player][ItemName]
end

function PlayerDataHandler_module:AddToInventory(Player, ItemName, Amount)
    local PlayerDecodedInventory = Http:JSONDecode(SessionData[Player][1])

    if module_Useful.FindItem(ItemName)then --< If the item name is in the _Replicated folder
        for _, v in pairs(PlayerDecodedInventory) do
            if v[1] == ItemName then --< Checks first value on table for item name Ex:) ["ItemName", Amount]
                v[2] = v[2] + 1
                SessionData[Player][1] = Http:JSONEncode(PlayerDecodedInventory)
            end
        end
    end

end

function PlayerDataHandler_module:ChangeStat(player, statName, changeValue)
    if SessionData[player][statName] == nil or 0 then
        SessionData[player][statName] = 0 + changeValue
    else
        SessionData[player][statName] = SessionData[player][statName] + changeValue
    end
end

local function SavePlayerData(Player) --<Saves player data
    local key = "--<" .. Player.userId
    local Success, Message = pcall(function()
        PlayerData:SetAsync(key, SessionData[Player])
    end)
    if not Success then
        print(Message)
    end

    PlayerData:SetAsync(key, SessionData[Player])
end

local function SetupPlayerData(Player) --<Sets up player data
    local key = "--<" .. Player.userId
    local data = PlayerData:GetAsync(key)
    if not data then
        local Inventory = {}
        SessionData[Player] = {Http:JSONEncode(Inventory), Money = 0}
        PlayerData:SetAsync(key, SessionData[Player])
        print("Player Set Up!")
    end
    SessionData[Player] = data
end

game.Players.PlayerRemoving:connect(function(Player)
    SavePlayerData(Player)
    SessionData[Player] = nil
end)


local function AutoSave()
    while wait(AUTOSAVE_INTERVAL) do
        for player, data in pairs(SessionData) do
            SavePlayerData(player)
        end
    end

end

game.Players.PlayerAdded:connect(SetupPlayerData)

return PlayerDataHandler_module

Code from Script in ServerScriptService

local PlayerStatManager = require(game.ServerStorage.PlayerDataHandler)

game.Players.PlayerAdded:connect(function(Player)
    local leaderstats = Instance.new("Model", Player)
    local MoneyValue = Instance.new("NumberValue", leaderstats)

    leaderstats.Name = "leaderstats"
    MoneyValue.Name = "Money"
    while wait(5) do
        PlayerStatManager:ChangeStat(Player, "Money", 3)
        MoneyValue.Value = PlayerStatManager[Player]["Money"]
    end
end)

Update: Seems to happen whenever I request a stat from the DataStore