Currently, you can hit an ArgumentNullException in case you are using the GraphSource feature of RadDiagram. The exception is thrown if you define a ContainerShapeStyle or ContainerShapeStyleSelector that adds a Style with a TargetType that is different than the one of the container added to the diagram. 

Throw a more descriptive error that tells the concrete reason. In this case, the issue relates to the wrong TargetType of the Style.

To reproduce the error, it is enough to add a container node (ContainerNodeViewModelBase<NodeViewModelBase>) in the graph source and then to add a ContainerShapeStyle with a wrong TargetType. For example, use a type of Button.