When I remove several nodes from diagram by calling diagram.Nodes.Remove(node), I get this error with some of them:

ERROR: System.NullReferenceException: Object reference not set to an instance of an object.
at MindFusion.Diagramming.Silverlight.DiagramItem.OnRemove()
at MindFusion.Diagramming.Silverlight.DiagramNode.OnRemove()
at MindFusion.Diagramming.Silverlight.Diagram.xe3e30df7c58d64c4(DiagramItem xccb63ca5f63dc470, Boolean xeff9019db96c96d8)
at MindFusion.Diagramming.Silverlight.RemoveItemCommand.Execute()
at MindFusion.Diagramming.Silverlight.x97f8b0311cc30886.xdf8cd2685f83d9b7(Command xfd6dd6e53b1a26d5)
at MindFusion.Diagramming.Silverlight.Diagram.RemoveItem(DiagramItem item)
at MindFusion.Diagramming.Silverlight.Diagram.xba535330911dfd91(Object xe0292b9ed559da7d, ItemEventArgs`1 xfbf34718e704c6bc)
at MindFusion.Diagramming.Silverlight.CollectionBase`2.RaiseRemoving(T item)
at MindFusion.Diagramming.Silverlight.CollectionBase`2.x52b190e626f65140(T xccb63ca5f63dc470, Boolean x808547253555a9ba)
at MindFusion.Diagramming.Silverlight.CollectionBase`2.Remove(T item)

Seems like a bug...