Add forgotten AddressFieldItemList.php

src/Plugin/Field/FieldType/AddressFieldItemList.php

+<?php
+
+namespace Drupal\address\Plugin\Field\FieldType;
+
+use Drupal\Core\Field\FieldItemList;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Defines the 'address' field item list class.
+ */
+class AddressFieldItemList extends FieldItemList {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultValuesFormValidate(array $element, array &$form, FormStateInterface $form_state) {
+    // Skip validation on the default value form.
+    // This allows setting an incomplete address as the default value.
+  }
+
+}