DialogView subclasses (such as "Add File") could sometimes cause JS

errors when they were closing. The issue here was a race between

multiple event handlers. Inside the modalBox implementation (which is

all our own jQuery-UI based code), a generic handler for clicks in the

button area would cause the modal box to self-destruct. This could then

conflict with the higher-level DialogView code which would additionally

try to call destroy. In previous versions of jQuery-UI this didn't

matter, but the version we're using now will error out when calling

methods on elements which are not widgets.
This change adds a check to make sure we don't try to destroy the

modalBox widget after it's already self-destructed.

Manually tested the "Add File" dialog.
Ran js-tests without any modalBox-related errors.