Resolve #3322452 "Warning undefined array"

Closes #3322452

src/Plugin/views/style/Table.php

@@ -671,7 +671,9 @@ class Table extends ViewsTable {
     // Get field from the relationship_entities, otherwise from the entity.
     if ($field_handler->options['relationship'] && $field_handler->options['relationship'] != 'none') {
       $relationship = $field_handler->options['relationship'];
-      $source = $result_row->_relationship_entities[$relationship];
+      if (isset($result_row->_relationship_entities[$relationship])) {
+        $source = $result_row->_relationship_entities[$relationship];
+      }
     }
     else {
       $source = $result_row->_entity;