The object that is managing semantics for this pipeline owner, if any.
An owner is created by ensureSemantics. The owner is valid for as long there are SemanticsHandles returned by ensureSemantics that have not yet been disposed. Once the last handle has been disposed, the semanticsOwner field will revert to null, and the previous owner will be disposed.
SemanticsOwner get semanticsOwner => _semanticsOwner;