ReferenceError: weakly-referenced object no longer exists

[MrBitPythoner]

The following code is search_book.py, which will be imported into main.py

Full Code:

from tkinter import *
from tkinter import messagebox
from tkinter import StringVar
import tkinter.ttk
import pyautogui as pg
import mysql.connector as mc

def search_book():
    global password, username
    usern = pg.prompt("Username", "Enter your username")
    passw = pg.password("Password", "Enter your password")
    try:
        conn = mc.connect(host="localhost", db="Library", user=usern, passwd=passw)
    except:
        messagebox.showerror("Incorrect Credentials", "Those credentials are incorrect")
        exit()

    global cur
    cur = conn.cursor()
    form = Tk()
    form.geometry("500x200")
    form.resizable(False, False)
    form.title("Search for a book")
    top_lbl = Label(form, text="Search For A Book", font="Courier 26").pack()
    
    OptionList = [
    "Pick A Search Type",
    "Title",
    "Author",
    "ISBN",
    ] 

    global variable
    variable = StringVar(form)
    variable.set(OptionList[0])

    search_dropd = OptionMenu(form, variable, *OptionList)
    search_dropd.pack()

    global search_entr
    search_entr = Entry(form, width=45)
    search_entr.pack()

    submit_btn = Button(text="SUBMIT", command=submit, width=34).pack()
        

def submit():
    search_words = search_entr.get()
    search_type = variable.get()
    if search_type == "Pick A Search Type":
        messagebox.showerror("Invalid Choice", "'Pick A Search Type' is not a valid search type.")
    else:
        global results
        if search_type == "ISBN":
            cur.execute("SELECT * FROM Books WHERE ISBN = %s", (search_words,))
            results = cur.fetchball()
        if search_type == 'Title':
            cur.execute("SELECT * FROM Books where ITLE = %s", (search_words,))
            results = cur.fetchball()
        if search_type == "Author":
            cur.execute("SELECT * FROM Books WHERE AUTHOR = %s", (search_words,))
            results = cur.fetchball()

        show_results(results)
    
def show_results(results_list):
    for i in range(len(results_list)):
        print(results_list[i])