fix bug when program create window instance many times

[sherry0429]

fix bug when

for i in range(0, 100):
    toaster.show_toast(...., duration=300)
    time.sleep(1)

and program create window instance many times

and remove destroy window after toast show. now window will be destroy after this:

toast.destroy()

or

with CustomToastNotifier() as toaster:
    toaster....

text codes:

import time

count = 0
with CustomToastNotifier() as toaster:
    while True:
        time.sleep(3)
        toaster.show_toast("Hello World!!!",
                           "Python is 10 seconds awsm!",
                           icon_path="custom.ico",
                           duration=100)
        count += 1
        if count >= 5:
            break