src/vs/workbench/api/common/extHostNotebookRenderers.ts/ExtHostNotebookRenderers/getOrCreateEmitterFor
class ExtHostNotebookRenderers:	private getOrCreateEmitterFor(rendererId: string) {
		let emitter = this._rendererMessageEmitters.get(rendererId);
		if (emitter) {
			return emitter;
		}

		emitter = new Emitter({
			onLastListenerRemove: () => {
				emitter?.dispose();
				this._rendererMessageEmitters.delete(rendererId);
			}
		});

		this._rendererMessageEmitters.set(rendererId, emitter);

		return emitter;
	}