There was an issue reported with NullReferenceException on SaveViewState about 6 months ago, and a fix was provided, however I am now getting this error when expanding on WebHierarchicalDataGrid.

Version:  15.2.20152.2042

This occurs after expanding and it seems to happen only after several items have been expanded, then the error occurs every time expand is done.  After the page is refreshed, expand works again even on the same records that previously produced errors. 

Adding:  the error occurs after changing a dataset parameter and doing an expand on the new records, however only if expand was done on the list before changing the parameter which is based on a drop-down.  So to reproduce:  Display page, expand a record, change the drop-down which displays the grid with new records then expand a record and the error occurs.  If you do not expand when the grid listing is based on the original parameter and then change the drop-down, the error does not occur when expanding on the new grid listing.

 
Exception information:
    Exception type: NullReferenceException
    Exception message: Object reference not set to an instance of an object.
   at Infragistics.Web.UI.GridControls.Filtering.SaveViewState()
   at Infragistics.Web.UI.Framework.ObjectBase.System.Web.UI.IStateManager.SaveViewState()
   at Infragistics.Web.UI.Framework.ObjectCollection.SaveViewStateItems()
   at Infragistics.Web.UI.Framework.ObjectCollection.SaveViewState()
   at Infragistics.Web.UI.Framework.ObjectCollection.System.Web.UI.IStateManager.SaveViewState()
   at Infragistics.Web.UI.GridControls.WebDataGrid.SaveViewState()
   at Infragistics.Web.UI.GridControls.ContainerGrid.SaveViewState()
   at Infragistics.Web.UI.GridControls.ContainerGrid.System.Web.UI.IStateManager.SaveViewState()
   at Infragistics.Web.UI.GridControls.WebHierarchicalDataGrid.SaveViewState()
   at System.Web.UI.Control.SaveViewStateRecursive(ViewStateMode inheritedMode)
   at System.Web.UI.Control.SaveViewStateRecursive(ViewStateMode inheritedMode)
   at System.Web.UI.Control.SaveViewStateRecursive(ViewStateMode inheritedMode)
   at System.Web.UI.Page.SaveAllState()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)