The DocsList.getRootFolder().getUrl() produces url which leads to such content for me:
Sorry, the page (or document) you have requested does not exist.
Make sure that you have the correct URL and that the owner of the document hasn't deleted it.
Is it bug?
In the Drive UI, the folders that return a valid URL also seem to have the drill-down option for ‘Share…’, which produces a URL that can be accessed by those with permissions. Since the root folder does not have such a drill-down option, I can understand why the URL Apps Script provides leads to such a message. Perhaps this behavior should be documented better — feel free to open an issue on the Issue Tracker to address this in the documentation.