I just got this when editing a node with a mobile browser:
Notice: Undefined index: fail in Drupal\views\Plugin\views\argument\ArgumentPluginBase->validate_argument() (line 964 of core/modules/views/lib/Drupal/views/Plugin/views/argument/ArgumentPluginBase.php).
Don't think it's related to #1877114: Arguments with relationships can't be configured on the first run.