QmlDesigner: Fix crash on shutdown

We have to set the correct onwership.
For objects from C++ QJSEngine::CppOwnership is the default,
unless the object comes from a function like here.

Change-Id: Idc35e2b06656da228a55d78e5b41f84795a8aa6e
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmldesigner/components/propertyeditor/qmlmodelnodeproxy.cpp

@@ -151,6 +151,8 @@ PropertyEditorSubSelectionWrapper *QmlModelNodeProxy::registerSubSelectionWrappe
         new PropertyEditorSubSelectionWrapper(node));
     m_subselection.append(wrapper);
 
+    QJSEngine::setObjectOwnership(wrapper.data(), QJSEngine::CppOwnership);
+
     return wrapper.data();
 }