Attempt to Index Local Field 'sessionData' (A Nil Value)?

I've expanded upon the saving player data tutorial using this code,

-- Setup table that we will return to scripts that require the ModuleScript.
local PlayerStatManager = {}
 
-- Create variable for the DataStore.
local DataStoreService = game:GetService('DataStoreService')
local playerData = DataStoreService:GetDataStore('CURRENTLYTESTING1')
 
-- Create variable to configure how often the game autosaves the player data.
local AUTOSAVE_INTERVAL = 60
 
-- Number of times we can retry accessing a DataStore before we give up and create
-- an error.
local DATASTORE_RETRIES = 3
 
-- Table to hold all of the player information for the current session.
local sessionData = {}
 
-- Function the other scripts in our game can call to change a player's stats. This
-- function is stored in the returned table so external scripts can use it.
function PlayerStatManager:ChangeStat(player, statName, changeValue)
	sessionData[player][statName] = sessionData[player][statName] + changeValue
end
 
-- Function to retry the passed in function several times. If the passed in function
-- is unable to be run then this function returns false and creates an error.
local function dataStoreRetry(dataStoreFunction)
	local tries = 0	
	local success = true
	local data = nil
	repeat
		tries = tries + 1
		success = pcall(function() data = dataStoreFunction() end)
		if not success then wait(1) end
	until tries == DATASTORE_RETRIES or success
	if not success then
		error('Could not access DataStore! Warn players that their data might not get saved!')
	end
	return success, data
end
 
-- Function to retrieve player's data from the DataStore.
local function getPlayerData(player)
	return dataStoreRetry(function()
		return playerData:GetAsync(player.UserId)
	end)
end
 
-- Function to save player's data to the DataStore.
local function savePlayerData(player)
	if sessionData[player] then
		return dataStoreRetry(function()
			return playerData:SetAsync(player.UserId, sessionData[player])
		end)
	end
end
 
-- Function to add player to the sessionData table. First check if the player has
-- data in the DataStore. If so, we'll use that. If not, we'll add the player to
-- the DataStore.
local function setupPlayerData(player)
	local success, data = getPlayerData(player)
	if not success then
		-- Could not access DataStore, set session data for player to false.
		sessionData[player] = false
	else
		if not data then
			-- DataStores are working, but no data for this player
			sessionData[player] = {FirstTimePlaying = true, TotalEggs = 0}
			savePlayerData(player)
			print(player.Name.." has been given first-time stats")
		else
			-- DataStores are working and we got data for this player
			sessionData[player] = data
			print(player.Name.." has been given back their stats")
		end
	end	
end
 
-- Function to run in the background to periodically save player's data.
local function autosave()
	while wait(AUTOSAVE_INTERVAL) do
		for player, data in pairs(sessionData) do
			savePlayerData(player)
		end
	end
end
 
-- Bind setupPlayerData to PlayerAdded to call it when player joins.
game.Players.PlayerAdded:connect(setupPlayerData)
 
-- Call savePlayerData on PlayerRemoving to save player data when they leave.
-- Also delete the player from the sessionData, as the player isn't in-game anymore.
game.Players.PlayerRemoving:connect(function(player)
	savePlayerData(player)
	sessionData[player] = nil
end)
 
-- Start running autosave function in the background.
spawn(autosave)
 
-- Return the PlayerStatManager table to external scripts can access it.
return {PlayerStatManager, sessionData}

and I attempt to call sessionData in this script

local result = PlayerStatManager.sessionData[player].FirstTimePlaying

but I get the error in the title. Does this mean the table doesn't array doesn't exist, if so, why?