Why would an error persist after after uninstalling the module? I created a module for Drupal 8 and when I try to add it to the menu it throws this error

 Uncaught PHP Exception Twig_Error_Runtime: "An exception has been thrown during the rendering of a template ("Route "hello_world.content" does not exist.") in "core/modules/toolbar/templates/menu--toolbar.html.twig" at line 36."


everything fell apart after this.  All of my site was unreachable because of an error page.

I tried removing the module and clearing the cache and the error is still there.  What do I have to do to fix this error?
This is 'normal' as your Twig files are being cached by Drupal. The easiest way to fix the issue is to run $ drush cache-rebuild (with Drush 8 which is the only release compatible with Drupal 8) which will handle all cache rebuild, including Twig cache. Alternatively, under sites/default/files/php you can safely delete the twig directory. If you're not in a multi web server environment, you can delete the php directory altogether, which will both delete the Twig cached files and the Service Container.