QmlDesigner: Fix crash

I found the crash when opening:
qtquickcontrols2\examples\quickcontrols2\contactlist\ContactForm.ui.qml.
The crash was triggered by the root item being a layout.

Change-Id: Id95aee6c76fbe1102578fb5a297b155645ebb157
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

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

@@ -371,6 +371,9 @@ bool itemIsMovable(const ModelNode &modelNode)
     if (modelNode.metaInfo().isSubclassOf("QtQuick.Controls.Tab"))
         return false;
 
+    if (!modelNode.hasParentProperty())
+        return false;
+
     if (!modelNode.parentProperty().isNodeListProperty())
         return false;