Exception in GridViewLayout.as:1053 occurs when data item is being changed by item renderer. The code:
is not null safe, because visibleItemRenderers vector can contain null items under some indices, made f.e. by code in GridViewLayout.as:1753 :
To reproduce this, create DataGrid with data provider, that contain items with Date fields, and create an ItemRenderer for that column with DateField component, that can change the date in source data item. Error will be throwed when data item's field will be changed and PropertyChangeEvent will be dispatched.