Fix against ticket #1581 - based on https://github.com/Joe7/gallery3/commit/0d36b97aca3b5ed53ea3ea398ce28b0f6198e577

modules/g2_import/helpers/g2_import.php

@@ -499,6 +499,9 @@ class g2_import_Core {
   static function set_album_highlight(&$queue) {
     // Dequeue the current album and enqueue its children
     list($g2_album_id, $children) = each($queue);
+    if (empty($children)) {
+      return;
+    }
     unset($queue[$g2_album_id]);
     foreach ($children as $key => $value) {
       $queue[$key] = $value;