Undefined array key "delete"

Closes #3416328

src/Plugin/Field/FieldWidget/InlineEntityFormComplex.php

@@ -1020,7 +1020,7 @@ class InlineEntityFormComplex extends InlineEntityFormBase implements ContainerF
     // has decided to delete.
     if ($entity_id) {
       $removed_reference = $remove_button['#removed_reference'];
-      if ($removed_reference === self::REMOVED_DELETE || ($removed_reference === self::REMOVED_OPTIONAL && $form_values['delete'] === 1)) {
+      if ($removed_reference === self::REMOVED_DELETE || ($removed_reference === self::REMOVED_OPTIONAL && !empty($form_values['delete']) && $form_values['delete'] === 1)) {
         $widget_state['delete'][] = $entity;
       }
     }