diff -urN ../roundcube-thunderbird_labels-1.6.1.orig/thunderbird_labels.php ./thunderbird_labels.php
--- ../roundcube-thunderbird_labels-1.6.1.orig/thunderbird_labels.php	2023-02-02 08:06:38.000000000 +0200
+++ ./thunderbird_labels.php	2023-04-13 18:56:55.998430000 +0300
@@ -427,15 +427,15 @@
 	{
 		$mbox_name = rcube_utils::get_input_value('_mbox', rcube_utils::INPUT_GPC); // appears to be the current one
 		$uids = rcube_utils::get_input_value('_uids', rcube_utils::INPUT_GPC);
-		if ($uids && $mbox_name)
+		if ($uids && $mbox_name && isset($params['folders'][0]))
 		{
 			$mbox_name = $params['folders'][0];
 			$RCMAIL = $this->rc;
 			# -- from here it's from check_recent.inc
 			$data = $RCMAIL->storage->folder_data($mbox_name);
 
-			if (empty($_SESSION['list_mod_seq']) || $_SESSION['list_mod_seq'] != $data['HIGHESTMODSEQ']) {
-			   $flags = $RCMAIL->storage->list_flags($mbox_name, explode(',', $uids), $_SESSION['list_mod_seq']);
+			if (empty($_SESSION['list_mod_seq']) || (isset($data['HIGHESTMODSEQ']) && $_SESSION['list_mod_seq'] != $data['HIGHESTMODSEQ'])) {
+			   $flags = $RCMAIL->storage->list_flags($mbox_name, explode(',', $uids), (empty($_SESSION['list_mod_seq']) ? null : $_SESSION['list_mod_seq']));
 			   foreach ($flags as $idx => $row) {
 				   $flags[$idx] = array_change_key_case(array_map('intval', $row));
 			   }