Fix fatal error in database_attachments plugin under some conditions (#1489726)

CHANGELOG

@@ -14,6 +14,7 @@ CHANGELOG Roundcube Webmail
 - Fix opening compose screen in new window after saving as draft (#1489643)
 - Fix message import dialog when no file is selected (#1489685)
 - Fix opening compose screen in new window after saving as draft (#1489643)
+- Fix fatal error in database_attachments plugin under some conditions (#1489726)
 
 RELEASE 1.0-rc
 --------------

plugins/database_attachments/database_attachments.php

@@ -126,8 +126,10 @@ class database_attachments extends filesystem_attachments
      */
     function cleanup($args)
     {
-        $cache = $this->get_cache();
-        $cache->remove($args['group'], true);
+        // check if cache object exist, it may be empty on session_destroy (#1489726)
+        if ($cache = $this->get_cache()) {
+            $cache->remove($args['group'], true);
+        }
     }
 
     /**