NullReferenceException is thrown when trying to save RadDiagram instance that contains a RadDiagramShape with its Geometry assigned to a GeometryGroup value. 

To work this around, you can flatten the GeometryGroup before setting the shape's Geometry property. This is done by calling the GetFlattenedPathGeometry() method of the GeometryGroup.

shape.Geometry = group.GetFlattenedPathGeometry();