isApplicable() on formatter is not overridden, causing an exception

Not sure why this is not failing any tests?

We don't have any tests for formatters right now.

+++ b/src/Plugin/Field/FieldFormatter/DynamicEntityReferenceEntityFormatter.php
@@ -24,4 +25,11 @@ use Drupal\entity_reference\Plugin\Field\FieldFormatter\EntityReferenceEntityFor
+    return TRUE;

So why always true?
Can't we do something like \Drupal::entityManager()->getDefinition($target_type)->hasViewBuilderClass(); here?

I guess we could loop over all enabled entity types and return TRUE. or all of them?

Well in this return TRUE make sense
Perhaps in #2377841: Allow user to chose per entity type formatter. we can change that.

Thank you for th bug report and patch committed and pushed.

Added major task #2405013: Add tests for all the formatters for formatter tests.