How to flatten column of lists when some contain Null values?

  flatten, pandas, python

This is likely a very obvious question, but I have a column containing lists that I am trying to flatten: i.e [AB, CD] -> AB, CD.

Sample dataframe:

data = [
    ["ABC", ["AB", "AB", "EF"]],
    ["DEF", ["CD", "EF"]],
    ["GHI", ["JK"]],

df = pd.DataFrame(data, columns=["ID", "list"])

I am applying this function to said column:

    flattened_string = ", ".join(map(str, string))
    return flattened_string
df = df["list_column"].apply(func)

However there are a few rows containing NaN values that initiate the error "TypeError: 'float' object is not iterable". Is there any way I can modify this function to fix this? Or perhaps (likely) a much better way to do this?

Thank you!!!

Source: Python Questions