scrolledtext widget error?

[shstein2002 at yahoo.com]

I wrote a twitter client which writes the statuses into a ScrolledText
widget, but it rarely gives a error that I can't nail down. The code
is:

twitwin.scrolled.config(state=NORMAL)
twitwin.scrolled.delete("1.0", END)
evens = True
_.links = []
for x in _.parser.tweets:
    start = _.scrolled.index(INSERT)
    print "start = ",start
    _.scrolled.insert(END, x['screen_name']+" ", ("bold",))
    txt = _.whitere.sub(" ",x['text'])  #the text with whitespace
compressed
    islink = False
    for y in _.urlre.split(txt):  #handle urls
        if y != "":
            if islink:
                a = _.scrolled.index(INSERT)
                _.scrolled.insert(END, y)#, ("link",))
                b = _.scrolled.index(INSERT)
                _.scrolled.tag_add("link", a, b)
                _.scrolled.tag_add("href%d"%len(_.links), a, b)
                _.links.append((a,b,y))  #save url info and position
in the widget
            else:
                _.scrolled.insert(END, y)
        islink = not islink
    _.scrolled.insert(END, "\n")
    if evens:
        _.scrolled.tag_add("light", start, END)  #alternating
background colors
    else:
        _.scrolled.tag_add("dark", start, END)
    evens = not evens
twitwin.scrolled.config(state=DISABLED)

The print statement usually prints a valid index, but rarely prints
"start=None" and it crashes. Should I be using something other than
INSERT or another method here?

Thanks in advance,
-- Rick