Tested with firefox.
I press the "external link button", I clik in the URL filed, I press backspace in order to remove the "http://" default text, but I press backspace 8 times instead of 7. Zzzzap, the dialog disappears.
Another problem : the cursor is not visible in the url field. Not important but a bit annoying.
Seems to have been fixed in Kupu 1.3.2. At least, I can't reproduce it.