Avoid dereferencing the source migration in the constructor.

src/Plugin/migrate/source/Migration.php

@@ -4,9 +4,9 @@
 
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\dgi_migrate\MigrationIterator;
-use Drupal\migrate\Plugin\migrate\source\SourcePluginBase;
 use Drupal\migrate\Plugin\MigrationInterface;
 use Drupal\migrate\Plugin\MigrationPluginManagerInterface;
+use Drupal\migrate\Plugin\migrate\source\SourcePluginBase;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -23,14 +23,14 @@ class Migration extends SourcePluginBase implements ContainerFactoryPluginInterf
    *
    * @var \Drupal\migrate\Plugin\MigrationPluginManagerInterface
    */
-  protected $migrationPluginManager;
+  protected MigrationPluginManagerInterface $migrationPluginManager;
 
   /**
-   * The target migration.
+   * Memoized target migration.
    *
    * @var \Drupal\migrate\Plugin\MigrationInterface
    */
-  protected $targetMigration;
+  protected MigrationInterface $targetMigration;
 
   /**
    * Constructor.
@@ -39,13 +39,12 @@ public function __construct(array $configuration, $plugin_id, $plugin_definition
     parent::__construct($configuration, $plugin_id, $plugin_definition, $migration);
 
     $this->migrationPluginManager = $migration_plugin_manager;
-    $this->targetMigration = $this->migrationPluginManager->createInstance($this->configuration['migration']);
   }
 
   /**
    * {@inheritdoc}
    */
-  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration = NULL) {
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, ?MigrationInterface $migration = NULL) {
     return new static(
       $configuration,
       $plugin_id,
@@ -55,18 +54,32 @@ public static function create(ContainerInterface $container, array $configuratio
     );
   }
 
+  /**
+   * Load and identify the target migration.
+   *
+   * @return \Drupal\migrate\Plugin\MigrationInterface
+   *   The target migration.
+   */
+  protected function getTargetMigration() : MigrationInterface {
+    if (!isset($this->targetMigration)) {
+      $this->targetMigration = $this->migrationPluginManager->createInstance($this->configuration['migration']);
+    }
+
+    return $this->targetMigration;
+  }
+
   /**
    * {@inheritdoc}
    */
   public function initializeIterator() {
-    return new MigrationIterator($this->targetMigration->getIdMap(), 'currentDestination');
+    return new MigrationIterator($this->getTargetMigration()->getIdMap(), 'currentDestination');
   }
 
   /**
    * {@inheritdoc}
    */
   public function getIds() {
-    return (array) $this->targetMigration->getDestinationPlugin()->getIds();
+    return (array) $this->getTargetMigration()->getDestinationPlugin()->getIds();
   }
 
   /**
@@ -81,7 +94,7 @@ public function fields() {
    */
   public function __toString() {
     return strtr('target migration: @migration', [
-      '@migration' => $this->targetMigration->id(),
+      '@migration' => $this->getTargetMigration()->id(),
     ]);
   }
 
@@ -106,13 +119,4 @@ public function __sleep() {
     return $vars;
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  public function __wakeup() {
-    parent::__wakeup();
-
-    $this->targetMigration = $this->migrationPluginManager->createInstance($this->configuration['migration']);
-  }
-
 }