Python showed that there is no attribute for one object

Python showed that there is no attribute for one object

I learned the book "programming python' these days. When I run the
examples, I met the problem. The shell showed me the error:
AttributeError: 'NoneType' object has no attribute 'pack'
However, I copy the exactly code from the book. I'm a freshman of Python.
I try to fix it by myself, but I failed. So I hope anyone could kindly
help me.
Thanks !!!!!!
CODEF
#File test.py
from tkinter import *
from tkinter.messagebox import showinfo
def MyGui(Frame):
def __init__(self, parent = None):
Frame.__init__(self, parent)
button = Button(self, text='press', command=reply)
button.pack()
def reply(self):
showinfo(title = 'popup',message ='Button pressed!')
if __name__ == '__main__':
window = MyGui()
window.pack()
window.mainloop()
#File test2.py
from tkinter import *
from test import MyGui
mainwin = Tk()
Label(mainwin,text = __name__).pack()
popup = Toplevel()
Label(popup,text = 'Attach').pack(side = LEFT)
MyGui(popup).pack(side=RIGHT)
mainwin.mainloop()