A javascript error could occur if the CloneSubfield function failed for
any reason. This would prevent the popup from disappearing.
Example : Importing informations of a UNIF_TITLE authority with multiple
$x subfields in the biblio's 440 field. 440$x isn't repeatable, so an
error occurs when trying to clone it.
http://bugs.koha-community.org/show_bug.cgi?id=6977 Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>