implement PropertyConstraintListItem::assignProperty to add item for each datum constraint

src/Gui/propertyeditor/PropertyModel.cpp

@@ -208,11 +208,11 @@ QModelIndex PropertyModel::propertyIndexFromPath(const QStringList& path) const
 
 void PropertyModel::buildUp(const PropertyModel::PropertyList& props)
 {
+    beginResetModel();
+
     // fill up the listview with the properties
     rootItem->reset();
 
-    beginResetModel();
-
     // sort the properties into their groups
     std::map<std::string, std::vector<std::vector<App::Property*> > > propGroup;
     PropertyModel::PropertyList::const_iterator jt;