Fixed script error after closing modal when using x-editable

Fixed a script error after closing a modal when using bootstrap-modal in combination with https://github.com/vitalets/x-editable.

X-editable uses jQuery.event.special.destroyed to implement a 'destroyed' event. This triggers the bootstrap-modal 'destroyed' event handler without the event parameter. See https://github.com/vitalets/x-editable/blob/master/dist/bootstrap-editable/js/bootstrap-editable.js#L1397

Solution: add a null check for the event parameter.

fixes #236