Tkinter python 3.x

# coding: utf-8

from tkinter import *
from tkinter import messagebox

class LoginWindow:
    def __init__(self, title):
        self.root = Tk()
        self.root.protocol("WM_DELETE_WINDOW", self.applicationExit)
        self.root.title(title)
        self.root.minsize(185, 80)
        frame_x, frame_y, frame_z = Frame().grid(row=0, column=0), Frame().grid(row=0, column=1), Frame().grid(pady=15, row=0, column=2)
        Label(frame_x, text="Usuário: ").grid(row=0, column=0)
        Label(frame_x, text="Senha: ").grid(row=1, column=0)
        self.user = Entry(frame_z, text="Usuário", width=20)
        self.user.grid(row=0, column=1)
        self.user.bind("<Return>", self.logar)
        self.password = Entry(frame_z, text="Senha", show='*', width=20)
        self.password.grid(row=1, column=1)
        self.password.bind("<Return>", self.logar)
        self.button_submit = Button(frame_y, text="Confirmar")
        self.button_submit.bind("<Button-1>", self.logar)
        self.button_submit.grid(row=2, column=1, pady=5)
        self.root.mainloop()

    def logar(self, event):
        user, passwd = self.user.get(), self.password.get()
        self.__user_required, self.__passwd_required = '123', '123'
        if len(user) > 1 and len(passwd) > 1:
            if user == self.__user_required:
                if passwd == self.__passwd_required:
                    messagebox.showinfo("Aviso", "Logado com sucesso!!")
                    self.applicationExit()
                    System('Alkos')
            else:
                messagebox.showerror("Erro", "Usuário e/ou senha incorreto(s)!")
        else:
            messagebox.showwarning("Aviso", "Digite algo")

    def applicationExit(self):
        self.root.destroy()

tempo = 10

class System:
    def __init__(self, title):
        self.root = Tk()
        self.root.protocol("WM_DELETE_WINDOW", self.applicationExit)
        self.root.title(title)
        self.frame = Frame().grid(row=0, column=0)
        Button(self.frame, text='Enviar míssil 23 ao pentágono', command=self.escrever).grid(row=0, column=1, pady=5, padx=5)
        self.root.mainloop()

    def escrever(self):
        global tempo
        Label(self.frame, text="Míssil 23 lançado!").grid(row=1, column=1)
        countdown_lb = Label(self.frame, text='COUNTDOWN INICIADO!')
        countdown_lb.grid(row=2, column=1)
        if tempo is None:
            tempo = tempo
        else:
            if tempo == 0:
                countdown_lb['text'] = 'O míssil explodiu com sucesso!'
                countdown_lb['fg'] = 'red'
            else:
                tempo -= 1
                countdown_lb.after(1000, self.escrever)
                countdown_lb['text'] = '%s segundos para explodir' % (tempo)

    def applicationExit(self):
        self.root.destroy()

LoginWindow('Login')