Fix crash when lauching dolphin with a search scheme
Summary: Widget objects are initialized in DolphinSearchBox::init(), which is called on QEvent::Polish, so we must set it visible before calling fromSearchUrl.
Test Plan: dolphin --new-window baloosearch://
Reviewers: Dolphin, elvisangelaccio
Subscribers: kfm-devel
Tags: Dolphin
Differential Revision: https://phabricator.kde.org/D24432