TKinter checkbox behaves different in different contexsts

I following code I create a window with just a checkbox and a button. When I get value of the checkbox by executing just the checkbox window, everything goes fine. When I construct that window from another window, checkbox behaves unexpectedly. Feel free to run following code. Result I get is:

1
0

I just don't get it why the second result is 0 and not 1. Any ideas?

import tkinter as tk
from tkinter import Tk, Button, Checkbutton

class SecondaryWindow:
    def __init__(self, window):
        self.window = window
        self.decimal_comma = tk.IntVar()

        Checkbutton(window, variable = self.decimal_comma).grid(row = 0, column = 0)
        Button(window, text = "Check the checkbox and click me", command = self.ok_pressed).grid(row = 1, column = 0)

    def ok_pressed(self, event = None):
        print(self.decimal_comma.get())
        self.window.destroy()

class MainWindow(Tk):
    def __init__(self):
        super().__init__()
        Button(self, text = "Click me", command = self.popup).pack(side=tk.LEFT)

    def popup(self, event = None):
        loader = SecondaryWindow(Tk())
        loader.window.wait_window()
        self.destroy()

def main1():
    MainWindow().mainloop()

def main2():
    top = Tk()

    loader = SecondaryWindow(top)
    loader.window.wait_window()

    top.mainloop()

if __name__ == "__main__":
    main2()
    main1()