Error message "This field may not be empty" doesn't appear on URL modal window

1. Go to any topic, press "Answer"
2. Press "URL" BB-Code button
3. Do not fill the Url Link field on the modal window.
4. Press OK
AR: no error message has been shown, the modal window disappears
ER: error message in red "This field may not be empty" should be shown, the modal window shouldn't disappear

The same behavior happens in PM