QmlDesigner: fix rare crash

Change-Id: Ifbafc1d54e8839be115a0b4dda90da09d4ac4ff5
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/studiowelcome/wizardhandler.cpp

@@ -39,8 +39,10 @@ void WizardHandler::destroyWizard()
     emit deletingWizard();
 
     m_selectedPreset = -1;
+    if (m_wizard) {
         m_wizard->deleteLater();
         m_wizard = nullptr;
+    }
     m_detailsPage = nullptr;
 }