Issue #3114494 by neelam_wadhwani, amykhailova: Undefined index hierarchy

src/Plugin/better_exposed_filters/filter/RadioButtons.php

@@ -48,7 +48,7 @@ class RadioButtons extends FilterWidgetBase {
       '#type' => 'checkbox',
       '#title' => $this->t('Add nested all/none selection'),
       '#default_value' => !empty($this->configuration['select_all_none_nested']),
-      '#disabled' => !$filter->options['expose']['multiple'] || !$filter->options['hierarchy'],
+      '#disabled' => (!$filter->options['expose']['multiple']) || (isset($filter->options['hierarchy']) && !$filter->options['hierarchy']),
       '#description' => $this->t('When a parent checkbox is checked, check all its children. If this option is disabled, edit the filter and check "Allow multiple selections" and edit the filter settings and check "Show hierarchy in dropdown".'
       ),
     ];