Use ICore::dialogParent for message boxes without parent so far

Change-Id: I2ee35796b2c041e6ea2ae6bd677e10167451d1b8
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/plugins/git/gerrit/gerritplugin.cpp

@@ -356,7 +356,7 @@ void GerritPlugin::openView()
 {
     if (m_dialog.isNull()) {
         while (!m_parameters->isValid()) {
-            QMessageBox::warning(0, tr("Error"),
+            QMessageBox::warning(ICore::dialogParent(), tr("Error"),
                     tr("Invalid Gerrit configuration. Host, user and ssh binary are mandatory."));
             const Id group = VcsBase::Constants::VCS_SETTINGS_CATEGORY;
             if (!ICore::showOptionsDialog(group, "Gerrit"))