For the exception that is reproducible in your project when you add both Shape and NodeViewModel (clicking first 2 buttons) - it is expected and you can reproduce it easily if you use both Items and ItemsSource collection of an ItemsControl - for example ListBox. This is a Framework limitaion for both WPF and Silverlight.
The other issue - the CircleShape is not properly saved / loaded - it is more complex and requires multiple changes in your code. However, most of them are based on this help article for serializing databound diagram
and we highly encourage you to read it completely. I changed your GraphSource to be SerializableGraphSoureBase
and added code for Serializing "Titulo" and "Subtitulo"in SerializeNode
. The I hit an issue that DeserializeNode is not fired in version 729 Q2 2014 SP. The good news is that this isse is resolved in Q1 2015 - the attached project uses this version (our latest official Release). We hope are able to upgrade to this version.