Found while working on completing #21363

When the stage subFusingMaterializer is used and the materialized stream throws an exception during materialization this will cause the ActorGraphInterpreter to leak, as the stage is added to newShells but never completes initialization.

Adding to that list happens in ActorGraphInterpreter.registerShell which is passed to the materializer as the subflowFuser parameter but I have not quite figured out why/how this happens even though there is an exception materializing.