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