How to convert argument passed to string with TextBlob?

I'm trying to learn how to scrape tweets using python. I'm trying to use the following code, but I keep getting the error. I'm not sure how to rectify it.

def fetch_tweets(query, count = 50):
    api = connect() # Gets the tweepy API object
    tweets = [] # Empty list that stores all the tweets
    try:
        fetched_data = api.search_tweets(q = query + ' -filter:retweets', count = count)
        for tweet in fetched_data:
            txt = tweet.text
            clean_txt = cleanText(txt) # Cleans the tweet
            stem_txt = TextBlob(stem(clean_txt)) # Stems the tweet
            sent = sentiment(stem_txt) # Gets the sentiment from the tweet
            tweets.append((txt, clean_txt, sent))
        return tweets
    except tweepy.TweepyException as e:
        print("Error: "+ str(e))
        exit(1)

tweets = fetch_tweets(query = 'Birdman', count = 200)
# Converting the list into a pandas Dataframe
df = pd.DataFrame(tweets, columns= ['tweets', 'clean_tweets','sentiment'])

# Dropping the duplicate values just in case there are some tweets that are copied and then stores the data in a csv file
df = df.drop_duplicates(subset='clean_tweets')
df.to_csv('data.csv', index= False)

ptweets = df[df['sentiment'] == 'positive']
p_perc = 100 * len(ptweets)/len(tweets)
ntweets = df[df['sentiment'] == 'negative']
n_perc = 100 * len(ntweets)/len(tweets)
print(f'Positive tweets {p_perc} %')
print(f'Neutral tweets {100 - p_perc - n_perc} %')
print(f'Negative tweets {n_perc} %')

I keep getting the following error

TypeError: The `text` argument passed to `__init__(text)` must be a string, not <class 'NoneType'>

And this is the stem(text) function, which is where the problem seems to occur:

def stem(text):
    porter = PorterStemmer()
    token_words = word_tokenize(text)
    stem_sentence = []
    for word in token_words:
        stem_sentence.append(porter.stem(word))
        return " ".join(stem_sentence)

I saw this response in a different place but since i'm new to coding, I wasn't sure how to use it?

df['data'].apply(lambda x: sentiment(' '.join(x)))