[Python] fignum_exists() not working on plt subplot

I am writing a program and have created the typical fig, ax = plt.subplot().

Now, I am in a loop and want to stop the program when the window is closed. When I do close the window, a new one opens instead.

I have not found a way to use not plt.fignum_exists().

What arguments do I need to pass this function to "trigger", when the window is closed?

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