Issue #3359841 Fixed PHP warning for non-existing array element

src/EventSubscriber/ExportLocalEntityMappingBase.php

@@ -202,10 +202,10 @@ abstract class ExportLocalEntityMappingBase implements EventSubscriberInterface
     $matching_field_info = NULL;
     $field_mapping = $sync->getFieldMapping();
     foreach ($field_mapping as $field_info) {
-      if ($field_info['machine_name'] !== $local_field) {
+      if (($field_info['machine_name'] ?? NULL) !== $local_field) {
         continue;
       }
-      if ($field_info['remote_name'] !== $remote_field) {
+      if (($field_info['remote_name'] ?? NULL) !== $remote_field) {
         continue;
       }