Errors with Data Persistence clothing scripts?

I'm trying to make a script where players are given no shirt and a certain pants ID the very first time they join the game, then when they buy a new shirt at the store it saves that they own that shirt and they spawn with that shirt, even if they leave the game. However, my script is outputting errors.

15:44:33.501 - ServerScriptService.PoorClothes:11: attempt to index field 'sessionData' (a nil value)

I'm also getting an error with SetStat but I couldn't get that one copy pasted

ModuleScript named PlayerStatManager

-- 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('PlayerData')

-- Create variable to configure how often the game autosaves the player data.
local AUTOSAVE_INTERVAL = 60

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

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] = {Shirt = "1", Pants = "624636052"}
            savePlayerData(player)
        else
            -- DataStores are working and we got data for this player
            sessionData[player] = data
        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

Saving script

-- Require ModuleScript so we can change player stats
local PlayerStatManager = require(game.ServerStorage.PlayerStatManager)

-- After player joins we'll periodically check and save their clothes
game.Players.PlayerAdded:connect(function(player)
    while wait(2) do
        local shirt = game.Workspace[player.Name].Shirt.ShirtTemplate:sub(14)
        local pants = game.Workspace[player.Name].Pants.PantsTemplate:sub(14)
        PlayerStatManager:SetStat(player, 'Shirt', shirt)
        PlayerStatManager:SetStat(player, 'Pants', pants)
    end
end)

Clothing updater script

local PlayerStatManager = require(game.ServerStorage.PlayerStatManager)

game.Players.PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function(character)   
        if character:findFirstChild("Shirt") ~= nil then
            character.Shirt.ShirtTemplate = "rbxassetid://" .. PlayerStatManager.sessionData[player].Shirt
        else
            local newshirt = Instance.new("Shirt")
            newshirt.Parent = character
            newshirt.Name = "Shirt"
            newshirt.ShirtTemplate = "rbxassetid://" .. PlayerStatManager.sessionData[player].Shirt
        end
        if character:findFirstChild("Pants") ~= nil then
            character.Pants.PantsTemplate = "rbxassetid://" .. PlayerStatManager.sessionData[player].Pants
        else
            local newPants = Instance.new("Pants")
            newPants.Parent = character
            newPants.Name = "Pants"
            newPants.PantsTemplate = "rbxassetid://" .. PlayerStatManager.sessionData[player].Pants
        end
    end)
end)