Window Not Appearing

[MGallo]

For some reason, everything on this works except for MainWindow(). It adds the person to the tempfile, but the MainWindow never pops up. I get no errors, it just won't show me the window :/

[inline]
from Tkinter import *
import os
import Tkinter as ttk
from ttk import *

creds = 'tempfile.temp'

def NewPerson():
global nameE
global shoeSizeE
global roots

roots = Tk()
roots.title('New Person')
intruction = Label(roots, text='Enter Information Below\n')
intruction.grid(row=0, column=0, sticky=E)

nameL = Label(roots, text='Name: ')
shoeSizeL = Label(roots, text='Shoe Size: ')
nameL.grid(row=1, column=0, sticky=W)
shoeSizeL.grid(row=2, column=0, sticky=W)

nameE = Entry(roots)
shoeSizeE = Entry(roots)
nameE.grid(row=1, column=1)
shoeSizeE.grid(row=2, column=1)

signupButton = Button(roots, text='Create', command=StorePerson)
signupButton.grid(columnspan=2, sticky=W)
roots.mainloop()

def StorePerson():
with open(creds, 'w') as f:
f.write(nameE.get())
f.write('\n')
f.write(shoeSizeE.get())
f.close()

roots.destroy()
MainWindow()

def MainWindow():

root = Tk()
root.title("AWD ShoeBot")

with open(creds) as f:
data = f.readlines()
name = data[0].rstrip()

mainframe = Frame(root)
mainframe.grid(column=0,row=0, sticky=(N,W,E,S) )
mainframe.columnconfigure(0, weight = 1)
mainframe.rowconfigure(0, weight = 1)
mainframe.pack(pady = 100, padx = 100)

tkvar = StringVar(root)

choices = { name }
tkvar.set(name)

popupMenu = OptionMenu(mainframe, tkvar, *choices)
Label(mainframe, text="Choose a Person").grid(row = 1, column = 1)
popupMenu.grid(row = 2, column =1)

def change_dropdown(*args):
print( tkvar.get() )

tkvar.trace('w', change_dropdown)

signupButton = Button(root, text='Create New Person', command=NewPerson)
signupButton.grid(columnspan=2, sticky=W)

root.mainloop()

if os.path.isfile(creds):
MainWindow()
else:
NewPerson()
[/inline]