QmlDesigner: Fix crash for property editor detach

Change-Id: I367b45c9ae0e8f18abffceb996dbf13a14d8089a
Reviewed-on: http://codereview.qt.nokia.com/1610Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>

src/plugins/qmldesigner/designercore/model/abstractview.cpp

@@ -463,7 +463,10 @@ void AbstractView::changeRootNodeType(const QString &type, int majorVersion, int
 
 ModelNode AbstractView::actualStateNode() const
 {
-    return ModelNode(m_model.data()->m_d->actualStateNode(), m_model.data(), const_cast<AbstractView*>(this));
+    if (model())
+        return ModelNode(m_model.data()->m_d->actualStateNode(), m_model.data(), const_cast<AbstractView*>(this));
+
+    return ModelNode();
 }
 
 } // namespace QmlDesigner