Limit source entities in settings to ones that have fields

Here's the patch

+++ b/src/Form/EntityUsageSettingsForm.php
@@ -113,8 +114,13 @@ class EntityUsageSettingsForm extends ConfigFormBase {
+      // If the entity type doesn't have fields it can't be a source, e.g. views
+      if (method_exists($entity_type->getOriginalClass(), 'getFields')) {
+        $source_options[$entity_type->id()] = $entity_type->getLabel();

I think we should be checking FieldableEntityInterface so something like:

      if ($entity_type->entityClassImplements(FieldableEntityInterface::class)) {
        $source_options[$entity_type->id()] = $entity_type->getLabel();
      }

You will need to add a use statement too.

FWIW I think this is a bug.

I think theoretically you could implement any entity tracking plugin that does not use fields BUT the entity track plugin definition has field_types so it is going to be quite hard.

But this potentially would break such a use-case. Maybe the way around this is to look at all the entity track plugins and check to see if the extend from \Drupal\entity_usage\EntityUsageTrackBase - if they all do then there we should not include non-fiedldable types. This would improve the UI for 99.5% of users.

Working on it.

Checking using FieldableEntityInterface seems to filter out the entity types without any fields from the source entities list. Created the MR !116 according to it.
I have not tried using the EntityUsageTrackBase as I am not clear of the idea of how to use it to check for entities without fields.