Undefined index when putting in a contribution thank-you date without a time
Notice: Undefined index: thankyou_date_time in CRM_Contribute_Form_AdditionalInfo::postProcessCommon() (line 275 of .../web/sites/all/modules/civicrm/CRM/Contribute/Form/AdditionalInfo.php).
It's straightforward but there's also a isNull() in there that seems odd so I'm wondering why it's there. It might be because sometimes it's the literal word 'null' and so you don't want to process that.
if (!empty($params['thankyou_date']) && !CRM_Utils_System::isNull($params['thankyou_date'])) {
$formatted['thankyou_date'] = CRM_Utils_Date::processDate($params['thankyou_date'], $params['thankyou_date_time']);
}