Do not assume that list of documents is non-empty
Summary: Host applications may not always have an active view, and a non-empty list of documents. This would currently crash when clicking the "Search"-button with "Current File" or "Open Files".
Test Plan: Tested with plugin loaded in RKWard, and no documents present. Segfault without patch, no matches, with patch.
Reviewers: cullmann, sars
Reviewed By: cullmann, sars
Subscribers: sars, kwrite-devel
Tags: Kate
Differential Revision: https://phabricator.kde.org/D26620