#12423 fix a focus change problem

The search widget triggers a showEvent() in updateTitle() leading to setting the focus to the default push button.
The check in updateTitle() for the need to restore the title avoids superfluous show events and avoids the unwanted focus change.

src/frontends/qt/GuiSearch.cpp

@@ -649,9 +649,10 @@ void GuiSearch::updateTitle()
 		// remove title bar
 		setTitleBarWidget(new QWidget());
 		titleBarWidget()->hide();
-	} else
+	} else if (titleBarWidget()) {
 		// restore title bar
 		setTitleBarWidget(nullptr);
+	}
 }