TypeError：预期的str，bytes或os.PathLike对象，而不是NoneType

Question

我正在尝试制作多重保存功能。这是我使用的代码 （我知道，global不是一个正确的编码，但无论如何）：

global game
game = None

global file
file = None

# stuff...

def saveGame():
    print('► Saving game...')
    with open(file, 'wb') as f:
        pickle.dump(PlayerIG, f, pickle.HIGHEST_PROTOCOL)
    print('► Game saved. You can access it by typing "load" at the title.')    

def loadGame():
    print('► Loading saved game...')
    if os.path.exists(file) == True: # TypeError on this line
        with open(file, 'rb') as f:
            global PlayerIG
            PlayerIG = pickle.load(f)        
        print('► Game loaded.')
        MainMenu()
    else:
        print('The savefile doesn\'t exist!')
        TitleScreen()


fileOpt = input()
if fileOpt == 'back':
    TitleScreen()
if fileOpt == '1' or '2' or '3' or '4':
    if fileOpt == '1':
        file = 'TGHA1.pickle'
    if fileOpt == '2':
        file = 'TGHA2.pickle'
    if fileOpt == '3':
        file = 'TGHA3.pickle'
    if fileOpt == '4':
        file = 'TGHA4.pickle'            
    if os.path.exists(file) == True:
        # code...

        subFileOpt = input()
        if subFileOpt == 'load':
            loadGame()
        if subFileOpt == 'delete':
            # code...

            delFileOpt = input()
            if delFileOpt == 'yes':
                print('► Deleting file...')
                os.remove(file)
                print('► File deleted!')
                TitleScreen()

# mainly invalid input stuff down from here...

问题是，当我转到# TypeError on this line的行（loadGame()函数也可能相同时，它会引发此错误：

TypeError: expected str, bytes or os.PathLike object, not NoneType

我知道这意味着什么，但我该如何解决？