[Tkinter] AttributeError: '' object has no attribute 'tk'

[Maryan]

I had to seperate this file from the main app, because had some errors that I had to fix them first. However, when I run the app I'm getting AttributeError. What am I missing?

from tkinter import *
from tkinter import Tk
from tkinter.ttk import Combobox
from tkcalendar import *
from tkinter import messagebox, filedialog
import sqlite3

class AddEmp():
    def __init__(self):
        ### FRAMES ###

        # Top Frame - Header
        self.topFrame = Frame(self, height = 100, bg = '#333')
        self.topFrame.pack(fill = X)

        # Center Frame - Buttons
        self.centerFrame= Frame(self, height = 750, bg = '#121212')
        self.centerFrame.pack(fill = X)

        # Top Frame - Data #
        # Header in Top Frame
        self.header_image = PhotoImage(file = 'img/add_employee.png')
        self.header_lbl = Label(self.topFrame, image = self.header_image, bg = '#333')
        self.header_lbl.grid(row = 0, column = 0, padx = 15, pady = 15)
        self.header_introduction = Label(self.topFrame, text = 'Add Employee', fg = '#b3b3b3', bg = '#333')
        self.header_introduction.grid(row = 0, column = 1)

        ######

root = Tk()
app = AddEmp()
root.title('All Fields are required')
root.geometry('440x823+700+150')
root.resizable(False, False)
root.mainloop()