Why does does commenting out line 44 fix this code?

wtf.py

import signal
from threading import Thread
import time


class MyThread(Thread):
    def __init__(self):
        super().__init__()
        self._stopped = False
    def run(self):
        while not self._stopped:
            time.sleep(0.2)
    def stop(self):
        self._stopped = True


class ThreadRunner:
    def start(self):
        self._my_thread = MyThread()
        self._my_thread.start()
    def end(self):
        self._my_thread.stop()
        self._my_thread.join()
        print('ThreadRunner end!')


def loop_forever():
    thread_runner = ThreadRunner()
    try:
        thread_runner.start()
        yield
    finally:
        print('loop_forever() is all done!')  # When does this line get run?
        thread_runner.end()


def listener():
    print('listener begin!')
    looper = loop_forever()
    next(looper)
    try:
        raise Exception()
    except Exception as e:
        es = e  # If you comment out this line, it doesn't hang
    print('listener... done!')


def main():
    def handle_exit(signum, *args): 
        raise Exception("SIGINT: EXITING")
    signal.signal(signal.SIGINT, handle_exit)
    listener()


if __name__ == "__main__":
    main()
    print('This program has exited. Or has it?')