Automatically close window after a certain time

The problem here is that mainloop() does not return until the GUI has shut down.

So, 30 seconds after the GUI has shut down and destroyed itself, you try to destroy it. And obviously that fails.

But you can't just move the sleep and destroy calls inside the main loop, because if you sleep in the middle of the main loop, the GUI will freeze up.

So, you need some kind of timer that will not stop the main loop. tkinter includes the after method for exactly that purpose. This answer gives a detailed example of using it.

Don't use time.sleep() with tkinter. Instead, call the function after on the widget you want to close.

Here it is the most simple example:

import tkinter as tk

w = tk.Tk()
w.after(30000, lambda: w.destroy()) # Destroy the widget after 30 seconds
w.mainloop()