Ошибка 'NoneType' object is not iterable в программе 'Phyton'

bag_of_fruits = ["apple","rottenBanana","apple"]
    
def remove_rotten(bag_of_fruits):
    bag_of_fruits =  [x.replace('rotten', '') for x in bag_of_fruits]
    return [x.lower() for x in bag_of_fruits]
 
print(remove_rotten(bag_of_fruits))

def remove_rotten(bag_of_fruits):
    if not bag_of_fruits:
        return []
    return [x.replace('rotten', '').lower() for x in bag_of_fruits]

def remove_rotten(bag_of_fruits):
    return [x.replace('rotten', '').lower() for x in bag_of_fruits] if bag_of_fruits else []

def remove_rotten(bag_of_fruits):
    return list(map(lambda x: x.replace("rotten", "").lower(), bag_of_fruits or []))

txt_htm = ('какой-то текст, большой и с абзацами') 
lst_htm = txt_htm.split('\n')
print(f'len lst_htm: {len(lst_htm)}')
lst_htm = list(set(lst_htm)) 
print(f'len lst_htm: {len(lst_htm)}')
lst_htm = lst_htm.sort()
print(f'lst_htm после sort():')
for ind, val in enumerate(lst_htm):
    print(ind, val)

    print(lst_htm[0])
          ~~~~~~~^^^
TypeError: 'NoneType' object is not subscriptable

txt_htm = ('какой-то текст, большой и с абзацами')
lst_htm = txt_htm.split('\n')
print(f'len lst_htm: {len(lst_htm)}')
lst_htm = list(set(lst_htm))
print(f'len lst_htm: {len(lst_htm)}')
lst_htm.sort()   #или lst_htm = sorted(lst_htm)
print(f'lst_htm после sort():')
print(lst_htm)
for ind, val in enumerate(lst_htm):
    print(ind, val)