[?] 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