Из-за чего окно программы при запуске может быть пусты, даже если код полностью написан (Python)?

from tkinter import *
class Application(Frame):
"""GUI-приложение, владеющее секретом долголетия!"""
def __init__(self, master):
"""Инициализирует рамку."""
super(Application, self).__init__(master)
self.grid()
self.create_widgets()

def create_widgets(self):
"""Создаёт кнопку, текстовое поле и текстовую область."""

self.inst_lbl = Label(self, text = "To find out the secret of longevity - enter the super secret code ((longevity)"
"Oh, damn, burned! Fuck! Okay, I hope no one notices!)")
self.inst_lbl.grid(row = 0, column = 0, columnspan = 2, sticky = W)
self.pw_lbl = Label(self, text = "Password: ")
self.pw_lbl.grid(row = 1, column = 0, sticky = W)
self.pw_ent = Label(self)
self.submit_bttn = Button(self, text = "find out the secret", command = self.reveal)
self.submit_bttn.grid(row = 2, column = 0, sticky = W)
self.secret.txt = Text(self, width = 35, height = 5, warp = WORD)
self.secret_txt.grid(row = 3, column = 0, columnspan = 2, sticky = W)

def reveal(self):
"""В зависимости от введённого пароля отвечает разными сообщениями."""
contents = self.pw_ent.get()
if contents == "longevity":
message = "To live to be 100 years, it is necessary first to live to 99" \
" and then be very careful!"
else:
message = "You entered the wrong password, " \
"so I can't share the secret with you!(("

self.secret_txt.insert(0.0, message)

root = Tk()
root.title("Longevity")
root.geometry("300x150")
app = Application(root)
root.mainloop()

Делал по книге