Python: How to remove process on background while using separate login window (tkinter)

Question

I have got a program with GUI (Tkinter) where I want to add a login system to. I programed a login window with a several functions in a separate library Login_v1. The program and de login system works well. However, it seems that a certain process is running on the background causing the computer to freeze (except for the program itself), while when I run both program or login library separately it does runs smoothly without running processes on the back. I think it has something to do with using two windows into one program and going from one window to another, but I don't know how to solve it.

Starting the main program it checks whether it should start the login window. When starting the login window it passes the function start_application through as command to execute in case of a succesfull login. The login window passes through to the main program start_application function a username and the account type.

Here you can find a relevant part of the code of the main program:

import tkinter as tk #GUI Library
import os #For file and directory handling
import Login_v1 as login #Login library


def start_application(Username, User_type):
    
    global mainWindow
    global FirstFrame
    global SecondFrame
    global ThirdFrame
    global USERNAME
    global USER_TYPE
    
    USERNAME = Username #Received from login
    USER_TYPE = User_type #Received from login
    
    load_config_settings() #Load config settings first
    load_all_settings(int(FILETYPE_CONFIG)) #Load all settings based on config
    
    root = tk.Tk()
    #Create a window as defined in the AppWindow class
    mainWindow = AppWindow(root)

    #Create a Frame as defined in class MainFrame
    FirstFrame = MainFrame(mainWindow)
    FirstFrame.grid(row=0, column=1, rowspan=2, sticky="NWES") #Positioning Frame on Window
    #Create a Frame as defined in class SearchFrame
    SecondFrame = SearchFrame(mainWindow)
    SecondFrame.grid(row=0, column=0, sticky="NWES") #Positioning Frame on Window
    #Create a Frame as defined in class ScrollFrame
    ThirdFrame = ScrollFrame(mainWindow)
    ThirdFrame.grid(row=1, column=0, sticky="NWES") #Positioning Frame on Window
    
    root.mainloop()


if __name__ == "__main__":
    if os.path.exists(USERS_FILE) == True:
        #Start login window 
        login = login.Login(icon="LegalNotesApp_icon.ico",
                            users_file=USERS_FILE,
                            modus="advanced",
                            command=lambda:start_application(login.username_entry_var.get(), login.account_type))
    else:

        start_application(Username="Administrator", User_type = "")

Here you can find the relevant part of the login library:

class Login(tk.Tk):
    def __init__(self,
                 title="Login",
                 command="pass",
                 accounts={},
                 users_file="",
                 modus="simple"):
        tk.Tk.__init__(self)
        self.resizable(0,0) #Window not resizable
        self.attributes("-topmost", 1) #Topmost window
        self.focus_force()
        
        
        #Passing through variables
        self.command = command #Action when login is succesful
        self.title(title) #Title of window
        self.users_dict = accounts #Dictonary of users & passes
        self.users_file = users_file #File to load/save dictonary of users & passes
        self.modus = modus #Account modus: simple, advanced, custom?


def login_event(self, *args):
        #Event when login button is pressed
        
        if self.users_dict != {}: #Check whether users are available
            
            #STEP 1: Check for empty fields
            if self.username_entry.get() != "" and self.password_entry.get() != "": #check wether username is not empty
            
                #STEP 2: Check for modus: "simple", "advanced", "custom"?
                if self.modus == "simple": #MODUS
                
                    if self.password_entry.get() == self.users_dict.get(self.username_entry.get()): #Check password
                        #Succesful login
                        self.destroy() #Login window not needed after login
                        self.command() #Action determined by user
                    

What is causing the process on the background? And how do I prevent it from happening? or what can I do better?