Issue #2826684 by toni4i, tuutti: Error When Performing Updates from 2.6 to 2.7

simple_sitemap.install

@@ -211,7 +211,9 @@ function simple_sitemap_update_8202() {
 function simple_sitemap_update_8203() {
   $old_config = $config = \Drupal::config('simple_sitemap.settings');
   foreach (['entity_types', 'custom'] as $config_name) {
-    $config = $old_config->get($config_name);
+    if (!$config = $old_config->get($config_name)) {
+      continue;
+    }
     \Drupal::service('config.factory')->getEditable("simple_sitemap.$config_name")
       ->setData($config)->save();
   }