Undefined index: group_option in temporarygroups_civicrm_validateForm() (line 56 of sites\default\files\civicrm\ext\temporarygroups-main\temporarygroups.php).

I get the above notice when I search contacts to add to the temporary group via /civicrm/group/search.

Solved it by changing the line 56.

Old: if ($form instanceof CRM_Contact_Form_Task_AddToGroup && $fields['group_option'])

New: if ($form instanceof CRM_Contact_Form_Task_AddToGroup && isset($fields['group_option']))