The object that is managing semantics for this pipeline owner, if any.
An owner is created by ensureSemantics or when the PipelineManifold to which this owner is connected has PipelineManifold.semanticsEnabled set to true. The owner is valid for as long as PipelineManifold.semanticsEnabled remains true or while there are outstanding SemanticsHandles from calls to ensureSemantics. The semanticsOwner field will revert to null once both conditions are no longer met.
When semanticsOwner is null, the PipelineOwner skips all steps relating to semantics.
SemanticsOwner? get semanticsOwner => _semanticsOwner;