"The "" plugin does not exist." – How do I discover the plugin when it is null?

I keep getting this in my logs:

DrupalComponentPluginExceptionPluginNotFoundException: The “” plugin does not exist. in DrupalCorePluginDefaultPluginManager->doGetDefinition() (line 59 of drupal-8.4.5/html/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php

Uncaught PHP Exception DrupalCoreEntityEntityStorageException: “The “” plugin does not exist.” at drupal-8.4.5/html/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php line 805.

I cannot pinpoint the cause because the $plugin_id value is empty.

How do I debug this?


Drupal version: 

Source: https://www.drupal.org/taxonomy/term/22/feed