On the highest level of a StateGraph, an instance of StateGraphRoot has to be present.
The StateGraphRoot object is needed, since all Step objects have an "outer" reference to communicate with the "nearest" CompositeStep (which inherits from PartialCompositeStep), especially to abort a CompositeStep via the "suspend" port. Even if no "CompositeStep" is present, on highest level a corresponding "inner" definition is needed and is provided by the StateGraphRoot object.