Fixes #259 by inverting the order of arguments to the event dispatcher

migrate_plus.module

@@ -83,5 +83,5 @@ function migrate_plus_migration_plugins_alter(array &$migrations) {
  * Implements hook_migrate_prepare_row().
  */
 function migrate_plus_migrate_prepare_row(Row $row, MigrateSourceInterface $source, MigrationInterface $migration) {
-  \Drupal::service('event_dispatcher')->dispatch(MigrateEvents::PREPARE_ROW, new MigratePrepareRowEvent($row, $source, $migration));
+  \Drupal::service('event_dispatcher')->dispatch(new MigratePrepareRowEvent($row, $source, $migration), MigrateEvents::PREPARE_ROW);
 }