TypeError: ' in <string> ' requires string as left operant, not None Type

import os
import random
import sys

words=[
'manzana',
'banano',
'pera',
'piña',
'papaya',
'sandia',
'cereza',
'granadilla',
'melon',
'coco',
'fresa',
'uva',
'mora',
'arandano',
'limon',
'naranja',
'mandarina',
'franbuesa'
]

def clear():
    if os.name == "nt":
        os.system("clear")
    else:
        os.system('clear')

def draw(bad_guesses, good_guesses, secret_word):
    clear()

    print('strikes: {}/7'.format(len(bad_guesses)))
    print('')

    for letter in bad_guesses:
        print(letter, end=" ")
    print('\n\n')

    for letter in secret_word:
        if letter in good_guesses:
            print(letter, end="")
        else:
            print('_', end="")

    print('')


def get_guess(bad_guesses, good_guesses):
    guess =input("Adivina una letra: ").lower()

    if len(guess) !=1:
        print("Solo puedes adivinar una letra")

    elif guess in bad_guesses or guess in good_guesses:
        print("ya haz probado con esa letra")

    elif not guess.isalpha():
        print("Solo puedes adivinar letras")

    else:
        return guess

def play(done):
    clear()
    secret_word = random.choice(words)
    bad_guesses = []
    good_guesses = []

    while True:
        draw(bad_guesses, good_guesses, secret_word)
        guess= get_guess(bad_guesses, good_guesses)

        if guess in secret_word:
            good_guesses.append(guess)
            found = True
            for letter in secret_word:
                if letter not in good_guesses:
                    found = False
            if found:
                print("GANASTE!")
                print("La palabra secreta era {}".format(secret_word))
                done = True
        else:
            bad_guesses.append(guess)
            if len(bad_guesses) == 7:
                draw(bad_guesses, good_guesses, secret_word)
                print("PERDISTE!")
                print("La palabra secreta era {}".format(secret_word))
                done = True

        if done:
            play_again = input("Quieres jugar de nuevo? y/n ").lower()
            if play_again != 'n':
                return play(done=False)
            else:
                sys.exit()

def welcome():
    start = input("Oprime enter para empezar o Q para salir  ").lower()
    if start == "q":
        print("Chao!")
        sys.exit()

print("Bienvenido a ahorcado!")

done= False

while True:
    clear()
    welcome()
    play(done)