UnboundLocalError in number_game exercise (Python Basic)

import random
import sys
import os

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

# create a function for guessing and condition for legit input
def get_guess(bad_guesses):
    clear()
    while True:
        try:
            guess = input('guess a number from 0 to 9: ')
            player_number = int(guess)
        except ValueError:
            print('you can only guess number.')
        else:
            # player guess more than 1 character:
            if len(guess) != 1:
                print('you can only guess one number!')
            # player guess a number second time
            elif player_number in bad_guesses:
                print('you already guess this number.')
        return player_number

# create function that tell player how many tries they have left & print out their bad guesses
def indicator(bad_guesses):
    print('Strike {}/5.'.format(len(bad_guesses)))
    for number in bad_guesses:
        print(number)
    print('')

# function for actual game
def number_game(done):
    clear()
    # let the machine pick a random number
    secret_number = random.randint(0, 9)
    # create a list for bad guesses
    bad_guesses = []
    # let ask the player
    while True:
        indicator(bad_guesses)
        guess = get_guess(bad_guesses)

        # if guess is legit, then consider whether the guess is correct or not
        # if the guess is correct
        if guess == secret_number:
            print('you win!')
            print('{} is the secret number.'.format(secret_number))
            done = True

        # if the guess is wrong
        else:
            bad_guesses.append(guess)
            # if guess is lower than secret number
            if guess < secret_number:
                print('the secret number is greater than {}.'.format(guess))
            # if guess is greater than secret number
            elif guess > secret_number:
                print('the secret number is lower than {}.'.format(guess))
            # if player fail to guess the right number in 5 turns
            if len(bad_guesses) == 5:
                print('you lose!')
                print('the secret number is {}.'.format(secret_number))
                done = True

        if done:
            while True:
                play_again = input('play again? Y/n ').lower()
                if play_again == 'y':
                    return number_game(done=False)
                elif play_again == 'n':
                    sys.exit()
                elif play_again != 'y' and play_again != 'n':
                    continue


def welcome():
    print('Hello, welcome to number game.')
    start = input("press enter/return to play or 'Q' to quit: ").lower()
    if start != 'q':
        number_game(done)
    else:
        print('Bye Bye!')
        sys.exit()

done = False
welcome()