The following exception is observed in certain cases:
at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.FlatLayoutStrategy.RealizeMergedCells(Double frozenOffset, IEnumerable`1 mergedCells)
at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.FlatLayoutStrategy.CalculateMergedCells(Boolean shouldInvalidateMeasure)
at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.CalculateMergedCells(Boolean shouldInvalidateMeasure)
at Telerik.Windows.Controls.GridView.GridViewCellsPanel.ArrangeOverride(Size finalSize)
When you have rows grouped by null value and you try to move one of the rows to another group, a NullReferenceException is thrown.
This is reproducible only when DataView is used as ItemsSource of RadGridView.
To work this around, avoid using null values. Or avoid using DataView.