src/vs/workbench/api/browser/mainThreadComments.ts/MainThreadCommentController/getKnownThread
class MainThreadCommentController:	private getKnownThread(commentThreadHandle: number): MainThreadCommentThread {
		const thread = this._threads.get(commentThreadHandle);
		if (!thread) {
			throw new Error('unknown thread');
		}
		return thread;
	}