Starts writing the next list of inner objects.
WriteInnerObjects can only be called, when all properties of the object itself have been written. It is only called for inner objects that are not known to the NShape framework as separate entities. For example the text items within a UML class shape are inner objects of the shape, however the shapes of a diagram or no inner objects.
When WriteInnerObjects is called the first time for the owning object, it deletes all inner objects of the property type for this owning object.