Added Subform validation.

src/Plugin/Field/FieldWidget/ViewsReferenceTrait.php

@@ -212,10 +212,21 @@ trait ViewsReferenceTrait {
   public static function validateDisplayId(array $field_values, FormStateInterface $form_state, string $key): void {
     // The select widget nests the target ID which is only later
     // fixed in the massaging of form values.
+    $message = t('Views Reference Display ID is required.');
     if (!empty($field_values[0]['target_id']) && empty($field_values[0]['display_id'])) {
-      $message = t('Views Reference Display ID is required.');
       $form_state->setErrorByName($key . '][0][display_id', $message);
     }
+    else {
+      foreach ($field_values as $subKey => $field_value) {
+        if (isset($field_value['subform'])) {
+          foreach ($field_value['subform'] as $parentKey => $value) {
+            if (isset($value[0]['target_id']) && isset($value[0]['display_id']) && empty($value[0]['display_id'])) {
+              $form_state->setErrorByName($key . '][' . $subKey . '][subform][' . $parentKey . '][0][display_id', $message);
+            }
+          }
+        }
+      }
+    }
   }
 
   /**