diff -urN ../html5_notifier-0.6.4.orig/html5_notifier.php ./html5_notifier.php
--- ../html5_notifier-0.6.4.orig/html5_notifier.php	2020-01-23 16:23:42.000000000 +0200
+++ ./html5_notifier.php	2023-03-16 15:00:40.773939000 +0200
@@ -50,7 +50,12 @@
 		$RCMAIL->storage->set_folder($args['mailbox']);
 		$RCMAIL->storage->search($args['mailbox'], $search, null);
 		$msgs = (array) $RCMAIL->storage->list_messages($args['mailbox']);
-		$excluded_directories = preg_split("/(,|;| )+/", $RCMAIL->config->get('html5_notifier_excluded_directories'));
+		$html5_notifier_excluded_directories = $RCMAIL->config->get('html5_notifier_excluded_directories');
+		if ((!isset($html5_notifier_excluded_directories)) or ($html5_notifier_excluded_directories === null)) {
+			$excluded_directories = [];
+		} else {
+			$excluded_directories = preg_split("/(,|;| )+/", $html5_notifier_excluded_directories);
+		}
 
 		foreach ($msgs as $msg) {
 		    $from = $msg->get('from');