MTP: fix runtime warning when showing a warning to the user.
Summary:
KIO says the following:
listDir() did not call finished() or error()! Please fix the "kio_mtp" KIO slave
and message boxes are annoying anyway, better use the error mechanism
for actual errors.
Test Plan: Not fully tested I admit, but seems obvious :-)
Reviewers: akrutzler, elvisangelaccio, apol
Reviewed By: akrutzler, elvisangelaccio, apol
Subscribers: kde-frameworks-devel, kfm-devel
Tags: Dolphin, Frameworks
Differential Revision: https://phabricator.kde.org/D16674