D18397: Fix crash on canceling the file open dialog

[Albert Astals Cid]

aacid added a subscriber: apol.

REPOSITORY
  R223 Okular

REVISION DETAIL
  https://phabricator.kde.org/D18397

To: vkrause
Cc: apol, nicolasfella, okular-devel, tfella, ngraham, darcyshen, aacid