How do I fix this high school python+mysql project?

Question

This project is an Inventory Management system. I am still a beginner. My code has a lot of issues, which I know. Right now I am struggling with using two different classes together (I also think I don't need to use classes if I can't organizing my code which is why I opted for it in the first place).

from tkinter import *
import customtkinter as ctk
from CTkMessagebox import CTkMessagebox
import mysql.connector

class Inventory():
    conn = mysql.connector.connect(host='localhost', user='root', password='1810', database='inventory')
    csr = conn.cursor()
    Logged_in = False
    #Initialization of onject variables
    def __init__(self):
        #Inheritance
        super().__init__()
        
        self.login = ctk.CTk()
        self.login.title('Inventory Management')
        self.login.geometry("800x600+10+10")
        self.login.resizable(False,False)

        #Username input
        self.usrname_label = ctk.CTkLabel(self.login, text="Username--", font=("Helvetica", 13))
        self.usrname_label.place(x=145,y=110)
        self.usrnm_input = ctk.CTkEntry(self.login, bg_color='white', fg_color='#AFE8DF', border_width=5)
        self.usrnm_input.place(x=230, y=110)

        #password input
        self.pswd_label = ctk.CTkLabel(self.login, text="Password--", font=("Helvetica", 13))
        self.pswd_label.place(x=145,y=150)
        self.pswd_input = ctk.CTkEntry(self.login, bg_color='white', fg_color='#AFE8DF', border_width=5, show='*')
        self.pswd_input.place(x=230, y=150)

        #buttons
        self.login_btn = ctk.CTkButton(self.login, text="LOGIN", bg_color='white', fg_color='blue', command= self.check )
        self.login_btn.place(x=230, y=200)
        self.welcm= Label(self.login, text="Welcome to Inventory Management", fg='#E6A405', font=("Helvetica", 25))
        self.welcm.place(x=140, y=40)
        self.fgt_btn = ctk.CTkButton(self.login, text="Forgot Password",bg_color='#00A7E5', fg_color='#00081F', command = self.forgot_passward )
        self.fgt_btn.place(x=280, y=400)
        self.sin_up = ctk.CTkButton(self.login, text="New User?", bg_color='#00A7E5', fg_color='#00081F', command = self.sign_up )
        self.sin_up.place(x=430, y=400)

        #Looping the login window
        self.login.mainloop()


    #to check login details
    def check(self):
        username = self.usrnm_input.get().strip()
        pswd = self.pswd_input.get()
        if username == "":
            no_user = CTkMessagebox(
                title="Credentials Error !",
                message="Please enter login details",
                option_1="Retry",
                icon = "cancel"
            )
        else:
            Inventory.csr.execute("SELECT * FROM Users WHERE Username = '{}'".format(username))
            user = Inventory.csr.fetchone()
        if user:
            if pswd in user:
                msg = CTkMessagebox(title = "Login", message="Login Successful !", option_1 = "Ok")
                if msg.get() == "Ok":
                    Inventory.Logged_in = True
                    self.login.withdraw()
                    self.usrnm_input.delete(0, END)
                    self.pswd_input.delete(0, END)
                    self.start_menu()
            else:
                inc_pswd = CTkMessagebox(
                title="Credentials Error !",
                message="Wrong password",
                option_1="Retry",
                icon = "cancel"
            )
        else:
            self.inc_usr()

#For Forgot Password Page
    def forgot_passward(self):
        self.fgt_window = ctk.CTk()
        self.fgt_window.geometry('800x600')
        self.fgt_window.title('Forgot Password')
        self.funam_lbl = ctk.CTkLabel(self.fgt_window, text="Username--", font=("Helvetica", 13))
        self.funam_lbl.place(x=165,y=110)
        self.funam_txt = ctk.CTkEntry(self.fgt_window, border_width=5, fg_color='#AFE8DF')
        self.funam_txt.place(x=250, y=110)
        self.fnam_lbl = ctk.CTkLabel(self.fgt_window, text="Name--", font=("Helvetica", 13))
        self.fnam_lbl.place(x=165,y=75)
        self.fnam_txt = ctk.CTkEntry(self.fgt_window, border_width=5, fg_color='#AFE8DF')
        self.fnam_txt.place(x=250, y=75)
        self.fsk_lbl = ctk.CTkLabel(self.fgt_window, text="Security Key--", font=("Helvetica", 13))
        self.fsk_lbl.place(x=165,y=145)
        self.fsk_txt = ctk.CTkEntry(self.fgt_window, border_width=5, fg_color='#AFE8DF')
        self.fsk_txt.place(x=250, y=145)
        self.fnxt_btn = ctk.CTkButton(self.fgt_window, text="Next",bg_color='#00A7E5', fg_color='#00081F', command = self.forgot_pswd_check)
        self.fnxt_btn.place(x=252, y=200)
        self.fgt_window.mainloop()
        
#For checking details for forgot password window
    def forgot_pswd_check(self):
        username = self.funam_txt.get().strip()
        s_key = int(self.fsk_txt.get().strip())
        name = self.fnam_txt.get().strip()
        if username == "" or s_key == "" or name == "":
            no_user = CTkMessagebox(
                title="Credentials Error !",
                message="Pleaase enter login details",
                option_1="Retry",
                icon = "cancel"
            )
            user = False
        else:
            Inventory.csr.execute("SELECT * FROM Users WHERE Username = '{}'".format(username))
            user = Inventory.csr.fetchone()
            print("\n\n\n"+str(user)+"\n\n\n")
        if user:
            if (name == user[1]) and (s_key == user[3]):
                self.frgt_pswd_user = user
                self.cre_vrf()
            elif s_key not in user:
                self.inc_sk()
            else:
                msg = CTkMessagebox(
                    title="Incorrect credentials!", message="Incorrect Name",
                            option_1="Ok"
                )
        else:
            self.inc_usr()

    #Verification of credentials
    #After checking from database returns the message after 
    def cre_vrf(self):
        cr_vrf = CTkMessagebox(title="Change Password", message="Credentials Verified !", option_1="Cont...")
        if cr_vrf.get()=="Cont...":
            self.fgt_window.destroy()
            self.Paswd_cng()
    #cont... button takes to change password page
        
        
    #To change password
    def Paswd_cng(self):
        self.pswd_cng = ctk.CTk()
        self.pswd_cng.geometry('600x600')
        self.pswd_cng.title('Change Password')
        self.fpw_lbl = ctk.CTkLabel(self.pswd_cng, text="Password--", font=("Helvetica", 13))
        self.fpw_lbl.place(x=125,y=145)
        self.new_pswd_entry = ctk.CTkEntry(self.pswd_cng, border_width=5, fg_color='#AFE8DF')
        self.new_pswd_entry.place(x=250, y=145)
        self.retype_pswd = ctk.CTkLabel(self.pswd_cng, text="Retype Password--", font=("Helvetica", 13))
        self.retype_pswd.place(x=125,y=180)
        self.retype_pswd_entry = ctk.CTkEntry(self.pswd_cng, border_width=5, fg_color='#AFE8DF')
        self.retype_pswd_entry.place(x=250, y=180)
        self.fpw_btn = ctk.CTkButton(self.pswd_cng, text="Cont...",bg_color='#00A7E5', fg_color='#00081F', command=self.cng_psd_prcs)
        self.fpw_btn.place(x=250, y=230)
        self.pswd_cng.mainloop()
        

    def cng_psd_prcs(self):
        psd = self.new_pswd_entry.get()
        if psd == self.retype_pswd_entry.get():
            Inventory.csr.execute("UPDATE Users SET pswd = '{}' WHERE Username = '{}'".format(psd, self.frgt_pswd_user[0]))
            Inventory.conn.commit()
            self.Paswd_ssfl()
        else:
            msg = CTkMessagebox(title = "Retype passwords", cancel = "Passwords Do not match", option_1="Ok")

    #To notify password change successful
    def Paswd_ssfl(self):
        paswd_sf = CTkMessagebox(title="Password Change!", message="Password Change successful, Please",option_1="Login")
        if paswd_sf.get() == "Login":
            self.pswd_cng.destroy()


    #For Sign Up page 
    def sign_up(self):
        self.sin_up = ctk.CTk()
        self.sin_up.geometry('600x600')
        self.sin_up.title('Sign Up')
        self.sunam_lbl = ctk.CTkLabel(self.sin_up, text="Username--", font=("Helvetica", 13))
        self.sunam_lbl.place(x=125,y=110)
        self.sunam_txt = ctk.CTkEntry(self.sin_up, border_width=5, fg_color='#AFE8DF')
        self.sunam_txt.place(x=250, y=110)
        self.snam_lbl = ctk.CTkLabel(self.sin_up, text="Name--", font=("Helvetica", 13))
        self.snam_lbl.place(x=125,y=75)
        self.snam_txt = ctk.CTkEntry(self.sin_up, border_width=5, fg_color='#AFE8DF')
        self.snam_txt.place(x=250, y=75)
        self.spw_lbl = ctk.CTkLabel(self.sin_up, text="Password--", font=("Helvetica", 13))
        self.spw_lbl.place(x=125,y=145)
        self.spw_txt = ctk.CTkEntry(self.sin_up, border_width=5, fg_color='#AFE8DF')
        self.spw_txt.place(x=250, y=145)
        self.srpw_lbl = ctk.CTkLabel(self.sin_up, text="Retype Password--", font=("Helvetica", 13))
        self.srpw_lbl.place(x=125,y=180)
        self.srpw_txt = ctk.CTkEntry(self.sin_up, border_width=5, fg_color='#AFE8DF')
        self.srpw_txt.place(x=250, y=180)
        self.snxt_btn = ctk.CTkButton(self.sin_up, text="Next",bg_color='#00A7E5', fg_color='#00081F', command=self.cre_svd )
        self.snxt_btn.place(x=252, y=240)
        self.sin_up.mainloop()
        
        
    #For credentials Saving
    def cre_svd(self):
        cre_svd = CTkMessagebox(title="Credentials Saved !", message="User data has been recorder", option_1="Cont...")
    #We need to connect this message box to cont_sinup page
        
        
    #For Continue Button, Sign Up page, To show security key
    def cont_sinup(self):
        cont_su = ctk.CTk()
        cont_su.geometry('600x600')
        cont_su.title('Confidential--Security Key')
        sk1_lbl= Label(cont_su, text="Please Save and Remember your Security Key", fg='#E6A405', font=("Helvetica", 25))
        sk1_lbl.place(x=55, y=40)
        #How to display random security key
        sk2_lbl= Label(cont_su, text="This Security Key will not be shown again", fg='#E6A405', font=("Helvetica", 16))
        sk2_lbl.place(x=180, y=500)
        sk_btn=ctk.CTkButton(cont_su, text="Cont..",bg_color='#00A7E5', fg_color='#00081F', command=self.sinup_cnfrm)
        sk_btn.place(x=248, y=440)
        cont_su.mainloop()
        
        
    #For Sign Up Confirmation:
    def sinup_cnfrm():
        siup_con = CTkMessagebox(title="Sign Up", message="Sign Up Confirmed, Please Restart the Application",option_1="Done")

    #For Incorrect Security Key
    #For Forgot Password Page
    def inc_sk(self):
        # Show some retry/cancel warnings
        incsk = CTkMessagebox(title="Incorrect credentials!", message="Security code incorrect",
                            option_1="Cancel", option_2="Retry")
        if incsk.get()=='Retry':
            show_Warning("Please try again")

    #Incorrect User 
    #For Forgot Password Page
    def inc_usr(self):
        incusr = CTkMessagebox(title="Incorrect credentials!", message="User Not Found",
                            option_1="Ok")

    #For Menu
    def start_menu(self):
        
        self.menu = Main()
        self.menu.title('Inventory Menu')
        self.menu.geometry("600x600+10+10")
        self.menu.resizable(False,False)

        self.menu.protocol("WM_DELETE_WINDOW", self.on_closing)

        self.menu.mainloop()
        
    
    def on_closing(self):
        msg = CTkMessagebox(title="o", message="Are you sure?", option_2="yes", option_1="no")
        if msg.get() == "yes":
            self.login.deiconify()
            self.menu.destroy()

    
class Main(ctk.CTk):

        def __init__(self):
            self.geometry("1000x800+50+50")
            self.title("Inventory Manager")

if __name__ == "__main__":
    App = Inventory()

After a successful Login, the login window should minimise using self.login.withdraw() and a new window of the main application should open. On closing that window, the login window should appear again.

The problem is that calling self.menu = Main() creates an infinite recursion loop somehow.

This obviously does not happen with the following block of code in place

def start_menu(self):
        
        self.menu = ctk.CTk()
        self.menu.title('Inventory Menu')
        self.menu.geometry("600x600+10+10")
        self.menu.resizable(False,False)

        self.menu.protocol("WM_DELETE_WINDOW", self.on_closing)

        self.menu.mainloop()

But this would make the code far more cluttered which makes using classes futile.