登录系统的代码中的Tkinter错误：AttributeError：'str'对象没有属性'tk'

如何解决登录系统的代码中的Tkinter错误：AttributeError：'str'对象没有属性'tk'

我是Python的新手，我正在尝试使用Tkinter设计一个简单的登录系统作为第一个小项目。这是我写的（我知道网格结构仍然很糟糕，但是我需要一些实践）：

from tkinter import *
from tkinter.ttk import *
from tkinter import ttk
import tkinter as tk
from PIL import ImageTk,Image



class Login_system():
    
    user = "user"
    passw = "pw"
    
    def __init__(self,root):


        self.root = root
        self.root.title("Login System Prova")
        self.entry = tk.Entry(root)
        self.root.geometry("500x400+400+400")  
        stvar = tk.StringVar()
        stvar.set("one")
        self.option = tk.OptionMenu(root,stvar,"one","two","three")
         
        frame = Frame(self.root)
        frame.grid(row = 0,column = 0,sticky = N)


        self.option = tk.OptionMenu(frame,"three")

        self.option.grid(row = 0,column = 1,sticky = NW)
        
        Label(frame,text = "Your Username").grid(row=1,column=0,sticky=NW,pady = 2)
        self.username = Entry()
        self.username.grid(row = 1,column=1,pady=2)

         
        Label(frame,text = "Your Password").grid(row=3,pady=2)
        self.password = Entry(show = "*")
        self.password.grid(row = 2,pady=2)
        
        
        ttk.Button(frame,text = "Login",command = self.login_user).grid(row = 6,pady=2)
        
        Checkbutton1 = Checkbutton(frame,text = "Keep me on-line").grid(row = 7,sticky = W,columnspan= 1)
        

    def login_user(self):

        if self.username.get() == self.user and self.password.get() == self.passw:

            root.destroy()

            #newroot = Tk()
            #application = ...(newroot)
            #newroot.mainloop()

        else:

            self.message = Label("Wrong Username or Password")
            self.message.grid(row = 6,column = 2)


if __name__ == "__main__":
    root = tk.Tk()
    Login_system = Login_system(root)
    root.mainloop()

如果我输入正确的初始化用户名和密码，该窗口将按预期消失，但是如果我输入了错误的用户名和密码，而收到消息“错误的用户名或密码”，则会出现此错误：

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Users\admin\AppData\Local\Programs\Python\Python38-32\lib\tkinter\__init__.py",line 1883,in __call__
    return self.func(*args)
  File "C:\Users\admin\Desktop\Python\GUI APP\login_system_new.py",line 75,in login_user
    self.message = Label("Wrong Username or Password")
  File "C:\Users\admin\AppData\Local\Programs\Python\Python38-32\lib\tkinter\ttk.py",line 759,in __init__
    Widget.__init__(self,master,"ttk::label",kw)
  File "C:\Users\admin\AppData\Local\Programs\Python\Python38-32\lib\tkinter\ttk.py",line 557,in __init__
    tkinter.Widget.__init__(self,widgetname,kw=kw)
  File "C:\Users\admin\AppData\Local\Programs\Python\Python38-32\lib\tkinter\__init__.py",line 2561,in __init__
    BaseWidget._setup(self,cnf)
  File "C:\Users\admin\AppData\Local\Programs\Python\Python38-32\lib\tkinter\__init__.py",line 2530,in _setup
    self.tk = master.tk
AttributeError: 'str' object has no attribute 'tk'

有人建议我该怎么避免吗？