There was two bugs:
* Process buffer was killed in ein:notebooklist-url-retrieve-callback
where should be handled by ein:query-ajax-callback. There was two
calls of kill-buffer. This is why the list buffer was killed.
* At the time ein:query-ajax-cancel-timer is called in
ein:query-ajax-callback the current buffer was changed because
pop-to-buffer is called in the callback. Wrapping callbacks
with save-current-buffer fixes the problem.