Add support for field_sample_value.

Closes #3402305

src/Controller/DropZones.php

@@ -190,7 +190,11 @@ class DropZones extends ControllerBase {
         if ($field->getFieldDefinition()->getFieldStorageDefinition()->isBaseField()) {
           continue;
         }
-        $field->generateSampleItems();
+        if ($this->moduleHandler()->moduleExists('field_sample_value')) {
+          \Drupal::service('field_sample_value.generator')->populateWithSampleValue($field);
+        } else {
+          $field->generateSampleItems();
+        }
       }
       $configuration['block_serialized'] = serialize($block_content);
     }