Fix crash in minibrowser

This is a workaround for bug QTBUG-42929, which causes QtWebEngine to
crash when going from about:blank to a valid URL in onEditingFinished.

Change-Id: Ia636da841de1e0460630f2ce435fc9fe8e19dabc
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>