Guard failing to load backend
Summary:
BackendManager::loadBackendPlugin can return a nullptr.
We currently use that pointer regardless.
The caller of this method ConfigOperationPrivate::loadBackend does
handle a nullptr and shows a relevant warning message.
BUG: 403568
Test Plan: Not done
Reviewers: Plasma, broulik
Reviewed By: Plasma, broulik
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D18508