Why does my Tkinter window crash when I try to display a label? - python-3.x

Question

I am using Pycharm and Python 3.8.
My Tkinter window stops responding and causes my program to crash.
Pycharm presents the following line in its output: Process finished with exit code -805306369 (0xCFFFFFFF)
I have the following code for displaying the progress of a for loop:
import tkinter as tk
from tkinter import filedialog, ttk
from tkinter.ttk import Progressbar
import json
class App(tk.Tk):
def __init__(self):
tk.Tk.__init__(self)
self.filename = ''
tk.Button(self, text = 'Upload File', command = self.get_file).grid(row = 0, column = 0)
tk.Button(self, text = 'Verify', command = self.verify).grid(row = 1, column = 0)
# tk.Label(self, text = 'some name').grid(row = 0, column = 1)
self.mainloop()
def get_file(self):
self.filename = tk.filedialog.askopenfilename(title = "Select File", filetypes = [("JSON", "*.json")])
tk.Label(self, text = self.filename).grid(row = 0, column = 1)
def verify():
lbl = tk.Label(self, text = 'Verifying Trustee Public Keys')
lbl.grid(row = 2, column = 0)
prog1 = ttk.Progressbar(self,
length = 100,
orient = "horizontal",
maximum = len(self.data["trustee_public_keys"]),
value = 0)
prog1.grid(row = 3, column = 0)
i = 1
for trustee_key_list in self.data['trustee_public_keys']:
print('Trustee :', i)
//some code
i = i+1
prog1['value'] = i
The get_file part works. But, when I click the verify button to execute the verify() function. The entire window freezes and I get a not responding message.
WHat is the problem with the following part of my code?
lbl = tk.Label(self, text = 'Verifying Trustee Public Keys')
lbl.grid(row = 2, column = 0)
What am I doing wrong?