pivot grid loads fine the first time (initial load).  When I click refresh (calls refresh method below) it crashes with the error above.

exposuregrid is not null.
exposuregrid.datasource is not null.
exposuregrid.datasource.itemssource is not null.
x is not null.

private void Refresh()
        {
            var x = ViewModel.GetExposureItems(); //returns observable collection
            ExposureGrid.DataSource = (IOlapViewModel)FindResource("FlatData");
            ((FlatDataSource)ExposureGrid.DataSource).ItemsSource = x; // crashes here on first refresh (but not initial load)
}

private void ExposureGrid_Loaded(object sender, RoutedEventArgs e)
{
 ExposureGrid.Loaded -= ExposureGrid_Loaded;

 Refresh();

 ExposureGrid.DataSource.Filters.Add(ExposureGrid.DataSource.CreateFilterViewModel(
 ExposureGrid.DataSource.Cube.Dimensions["PM_Name"].Hierarchies.First()));

 ExposureGrid.DataSource.Filters.Add(ExposureGrid.DataSource.CreateFilterViewModel(
  ExposureGrid.DataSource.Cube.Dimensions["DeliveryDate"].Hierarchies.First()));

 ExposureGrid.DataSource.Filters.Add(ExposureGrid.DataSource.CreateFilterViewModel(
  ExposureGrid.DataSource.Cube.Dimensions["IsFail"].Hierarchies.First()));

 FilterHelper.FilterOption("DeliveryDate", "All", false);
 FilterHelper.FilterOption("IsFail", "All", false);
 FilterHelper.FilterOption("IsFail", "N/A", true);
 OnDeliveryChanged(this, new EventArgs());
 OnPMFilterChanged(this, new EventArgs());
 chkIsNetFails_Click(this, new RoutedEventArgs());
}

   at Infragistics.Olap.FlatData.FlatDataFilterSource.OnSelectionChanged()
   at Infragistics.Olap.FilterSource.set_IsSelected(Nullable`1 value)
   at Infragistics.Olap.DataSourceBase.UnregisterFilterSource(IMember member)
   at Infragistics.Olap.FlatData.FlatDataSource.UnregisterFilterSource(IMember member)
   at Infragistics.Olap.DataSourceBase.ReleaseFilterSourceItems(ILevel level, Boolean refreshGrid)
   at Infragistics.Olap.DataSourceBase.UnregisterFilterSource(IFilterViewModel filterViewModel)
   at Infragistics.Olap.DataSourceBase.DetachAreaItemViewModel(IEnumerable`1 items)
   at Infragistics.Olap.DataSourceBase.OnFiltersChanged(NotifyCollectionChangedEventArgs args)
   at Infragistics.Olap.DataSourceBase.OnFiltersChanged(Object sender, NotifyCollectionChangedEventArgs e)
   at System.Collections.Specialized.NotifyCollectionChangedEventHandler.Invoke(Object sender, NotifyCollectionChangedEventArgs e)
   at Infragistics.Olap.SwapCollection`1.OnCollectionChanged(NotifyCollectionChangedEventArgs e)
   at Infragistics.Olap.SwapCollection`1.RemoveItem(Int32 index)
   at System.Collections.ObjectModel.Collection`1.RemoveAt(Int32 index)
   at Infragistics.Olap.SwapCollection`1.ClearItemsInternal(Boolean suppressMessages)
   at Infragistics.Olap.SwapCollection`1.ClearItems()
   at System.Collections.ObjectModel.Collection`1.Clear()
   at Infragistics.Olap.DataSourceBase.ClearAreaItemsCollections()
   at Infragistics.Olap.DataSourceBase.set_Cube(ICube value)
   at Infragistics.Olap.DataSourceBase.DataProviderLoadCubesCompleted(Object sender, LoadCubesCompletedEventArgs e)
   at Infragistics.Olap.DataProviderBase.OnLoadCubesCompleted(LoadCubesCompletedEventArgs args)
   at Infragistics.Olap.FlatData.FlatDataModelProvider.LoadCubesAsync(String catalogName)
   at Infragistics.Olap.DataSourceBase.LoadCubesAsync(String catalogName)
   at Infragistics.Olap.FlatData.FlatDataSource.OnSchemaLoaded(LoadSchemaCompletedEventArgs e)
   at Infragistics.Olap.DataSourceBase.DataProviderLoadSchemaCompleted(Object sender, LoadSchemaCompletedEventArgs e)
   at Infragistics.Olap.DataProviderBase.OnLoadSchemaCompleted(LoadSchemaCompletedEventArgs args)
   at Infragistics.Olap.FlatData.FlatDataModelProvider.LoadSchemaAsync()
   at Infragistics.Olap.FlatData.FlatDataModelProvider.ItemsSourceChanged(Object sender, ItemsSourceChangedEventArgs e)
   at Infragistics.Olap.FlatData.FlatDataConnectionSettings.OnItemsSourceChangedImpl(Object newValue, Object oldValue)
   at Infragistics.Olap.FlatData.FlatDataConnectionSettings.OnItemsSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
   at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
   at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
   at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
   at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType, Boolean isInternal)
   at System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value)
   at Infragistics.Olap.FlatData.FlatDataConnectionSettings.set_ItemsSource(IEnumerable value)
   at Infragistics.Olap.FlatData.FlatDataSource.set_ItemsSource(IEnumerable value)
   at Poolman.Views.ExposureMatrix.Refresh() in C:\svn_workspace\pool_man\src\Pool.man\Views\ExposureMatrix.xaml.cs:line 118