please help me remove error for string.strip()

[jamie_01]

#!/usr/bin/python3

from bs4 import BeautifulSoup
import requests
import lxml
import csv


URL = "https://sandiego.craigslist.org/search/sof"
page = requests.get(URL)
soup = BeautifulSoup(page.content, "lxml")

# h3 tag
job_heading = []
for a in soup.find_all("h3"):
    job_heading.append(a.string.strip())

# location
time = []
for b in soup.find_all("time"):
    time.append(b.string.strip())

with open('output.csv', 'w') as file:
    writer = csv.writer(file, delimiter=',')

    writer.writerow(["job_heading", "time"])

    for i in range(25):
        writer.writerow([
            job_heading[i],
            time[i]
        ])

please, help me remove this error so after running this script, there is no "AtributeError: 'NoneType' object has no attribute 'strip'" thanks.