I believe that with register, he means during the moment the server identifies your host, and acknowledges you as being connected to it.

If this is a bug, I think it would be that /list works halfway during this moment. It will start the process, but not open the window. If you then try to /list again, and the previous request is still running, it will naturally give an error, though the window remains closed. So maybe an extra check should be added somewhere, so /list can only be done when fully connected, or /list will look if the window is closed when a request is running.