Resolve #3395002 "Php warning when"

Closes #3395002

src/Plugin/Field/FieldFormatter/PatternOneForAllFormatter.php

@@ -172,7 +172,7 @@ class PatternOneForAllFormatter extends FieldWrapperBase implements ContainerFac
     // Set pattern fields.
     $fields = [];
     foreach ($mapping as $field) {
-      if ($field['destination'] === '_hidden') {
+      if (isset($field['destination']) && $field['destination'] === '_hidden') {
         continue;
       }