Issue #2872570 followup: Fix the default value handling.

config/schema/address.schema.yml

@@ -91,8 +91,7 @@ field.value.address_country:
       label: 'Country code'
 
 field.value.address_zone:
-  type: address_zone
-  label: 'Default value'
+  type: ignore
 
 field.field_settings.address:
   type: mapping

src/Element/Zone.php

@@ -113,7 +113,6 @@ class Zone extends FormElement {
       $element_state['territories'] = $element_state['territories'] ?: [NULL];
       self::setElementState($element['#parents'], $form_state, $element_state);
     }
-    $element['#required'] = TRUE;
 
     $element = [
       '#tree' => TRUE,

src/Plugin/Field/FieldType/ZoneItem.php

@@ -16,6 +16,7 @@ use Drupal\Core\TypedData\DataDefinition;
  *   label = @Translation("Zone"),
  *   description = @Translation("An entity field containing a zone"),
  *   category = @Translation("Address"),
+ *   list_class = "\Drupal\address\Plugin\Field\FieldType\ZoneItemList",
  *   default_widget = "address_zone_default",
  *   cardinality = 1,
  * )

src/Plugin/Field/FieldType/ZoneItemList.php

+<?php
+
+namespace Drupal\address\Plugin\Field\FieldType;
+
+use Drupal\Core\Field\FieldItemList;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Represents a list of zone item field values.
+ *
+ * Works around core not serializing/unserializing default values.
+ */
+class ZoneItemList extends FieldItemList {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applyDefaultValue($notify = TRUE) {
+    if ($default_value = $this->getFieldDefinition()->getDefaultValue($this->getEntity())) {
+      foreach ($default_value as $index => $value) {
+        $default_value[$index] = unserialize($value);
+      }
+      $this->setValue($default_value, $notify);
+    }
+    else {
+      parent::applyDefaultValue($notify);
+    }
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultValuesFormSubmit(array $element, array &$form, FormStateInterface $form_state) {
+    $default_value = parent::defaultValuesFormSubmit($element, $form, $form_state);
+    if ($default_value) {
+      foreach ($default_value as $index => $value) {
+        $default_value[$index] = serialize($value);
+      }
+    }
+
+    return $default_value;
+  }
+
+}