Basically the error is that c.get() has the value of '' which is not an int.

What you should do instead is to have a try except statement like this:

def convert():
  try:
    result =   int(c.get())  / 5 * 8 + 32 +  " Fahrenheit"
    r['text'] = result
  except:
    r['text'] = 'ERRRORRORORROOROROROROOROR'

that way you can also reliablely and gracefully handle bad input that is not number