Fix crash when loading file with attached properties

The metatype system doesn't know about attached properties,
and asserted when trying to call isListProperty().

Let's just assume that for such properties, "a : b" actually
means a is not of type QList.

Reviewed-by: Erik Verbruggen

src/plugins/qmldesigner/core/model/texttomodelmerger.cpp

@@ -391,7 +391,8 @@ void TextToModelMerger::syncNode(ModelNode &modelNode,
             } else {
                 if (isValidPropertyForNode(modelNode, astPropertyName)) {
                     AbstractProperty modelProperty = modelNode.property(astPropertyName);
-                    if (modelProperty.metaInfo().isListProperty()) {
+                    if (modelProperty.metaInfo().isValid()
+                        && modelProperty.metaInfo().isListProperty()) {
                         syncArrayProperty(modelProperty, QList<QmlJS::AST::UiObjectMember*>() << member, context, differenceHandler);
                     } else {
                         syncNodeProperty(modelProperty, binding, context, differenceHandler);