When you got error message like "Property null not found ***", try to fix it by the following way:
1)if you are using AdvancedDatagrid, make sure you only update the data in the data provider instead of setting a new reference to data provider each time.
(dgPortfolioViewer.dataProvider as ArrayCollection).removeAll();
(dgPortfolioViewer.dataProvider as ArrayCollection).addAll(value);
2) make sure the propery name in datafield is existed in your object
Property null not found