QmlDesigner: Fix crash in tests

This is a behaivour change and the crash seemes to be never
triggered by application code.

Change-Id: I25d986139c47fd1acd0989220942e6e7ec69ef5d
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>

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

@@ -336,7 +336,7 @@ void ModelNode::setParentProperty(NodeAbstractProperty parent)
         throw InvalidModelNodeException(__LINE__, __FUNCTION__, __FILE__);
     }
 
-    if (parent == parentProperty())
+    if (hasParentProperty() && parent == parentProperty())
         return;
 
     parent.reparentHere(*this);