I want to write a program to calculate time for datetime in tkinter module, but when calculating time, an error is thrown, how to fix it?

Question

Python

Gives the following error:

hour_end = int(enrty_hour_end.get())
ValueError: invalid literal for int() with base 10: ''

Here is the function:

def find_interval():
    hour_begin = enrty_hour_begin.get()
    int(hour_begin)
    minute_begin = enrty_minute_begin.get()
    int(minute_begin)
    hour_end = enrty_hour_end.get()
    int(hour_end)
    minute_end = enrty_minute_end.get()
    int(minute_end)
    a = timedelta(hours=hour_begin, minutes=minute_begin)
    b = timedelta(hours=hour_end, minutes=minute_end)
    c = b-a
    lbl_interval.config(text=c)