QmlDesigner: Fix crash

Change-Id: Iaabc04e97fb7e06d462e7716be27a24471be2bd2
Reviewed-by: Aleksei German <aleksei.german@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

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

@@ -944,6 +944,9 @@ bool ModelNode::hasNodeListProperty(const PropertyName &name) const
 
 static bool recursiveAncestor(const ModelNode &possibleAncestor, const ModelNode &node)
 {
+    if (!node.isValid())
+        return false;
+
     if (node.hasParentProperty()) {
         if (node.parentProperty().parentModelNode() == possibleAncestor)
            return true;