Squish: Avoid crash

The SquishControlBar becomes unexpectedly parent of the
critical error message box.
If the runner stops e.g. with a broken connection to the
server due to license issues the stop triggers a shutdown
of the server which in turn destructs the control bar, but
this crashes due to the message box.
Use the "real" main window instead which also corrects the
positioning of the message box.

Change-Id: I8ec3a100a51761169d8820175196dfc84b092410
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/squish/squishtools.cpp

@@ -338,7 +338,7 @@ void SquishTools::setState(SquishTools::State state)
                                                      QDir(m_suitePath).dirName(),
                                                      &error);
             if (!error.isEmpty())
-                QMessageBox::critical(Core::ICore::dialogParent(), Tr::tr("Error"), error);
+                QMessageBox::critical(Core::ICore::mainWindow(), Tr::tr("Error"), error);
             logrotateTestResults();
         } else {
             m_xmlOutputHandler->clearForNextRun();