[Tkinter] _tkinter.TclError: can't invoke "destroy" command: application has been destroyed

[knoxvilles_joker]

OK, I added root.focus_set() after defining the root window and the popup window after closing allows focus to remain on the main window and each child window.

root = tk.Tk ()

root.title ("Root Window")  
root.geometry ("512x256")  
root.focus_set()
label1 = Label (root, text = "This is the Root Window")
button = Button (root, text = "Open Toplevel Window")
button.config (command = open_a_toplevel_window)
button1 = Button (root, text = "Open Toplevel Windowb")
button1.config (command = open_b_toplevel_window)
button2 = Button (root, text = "Open Toplevel Windowc")
button2.config (command = open_c_toplevel_window)
label1.pack ()
button2.place (x = 50, y = 75)
button1.place (x = 210, y = 100)
button.place (x = 110, y = 50)
open_d_toplevel_window ()

root.mainloop ()