'NoneType' object has no attribute 'strip'

from bs4 import BeautifulSoup
import requests
import openpyxl
def get_html(url):
    try:
        r=requests.get(url)
        r.raise_for_status()
        r.encoding=r.apparent_encoding
        return r.text
    except:
        r="fail"
        return r
def get_contents(ulist,rurl):
    soup=BeautifulSoup(rurl,'html.parser')
    trs=soup.find_all('tr')
    for tr in trs:
        ui=[]
        for td in tr:
            ts=td.string
            if ts =='':
                continue
            ts=ts.strip()
            ui.append(ts)
        ulist.append(ui)
def saveList(ulist):
    wb=openpyxl.Workbook()
    ws=wb.active
    r=1
    for line in ulist:
        for col in range(1,len(line)+1):
            ws.cell(row=r,column=col).value=line[col-1]
        r=r+1
    wb.save("d:\\work.xlsx")
if __name__ =="__main__":
    urli=[]
    url="https://gdp.gotohui.com"
    rs=get_html(url)
    get_contents(urli,rs)
    saveList(urli)

报错

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-17-6fe05a5014a7> in <module>
     36     url="https://gdp.gotohui.com"
     37     rs=get_html(url)
---> 38     get_contents(urli,rs)
     39     saveList(urli)

<ipython-input-17-6fe05a5014a7> in get_contents(ulist, rurl)
     20             if ts =='':
     21                 continue
---> 22             ts=ts.strip()
     23             ui.append(ts)
     24         ulist.append(ui)

AttributeError: 'NoneType' object has no attribute 'strip'

        if ts  is '' or ts is None: 改一下试试吧，