(mvc/acl) validate before use of system tag

src/opnsense/mvc/app/models/OPNsense/Core/ACL.php

@@ -153,6 +153,7 @@ class ACL
 
         // gather user / group data from config.xml
         $config = Config::getInstance()->object() ;
+        if ($config->system != null) {
             foreach ($config->system->children() as $key => $node) {
                 if ($key == 'user') {
                     $this->legacyUsers[$node->name->__toString()] = array() ;
@@ -171,6 +172,7 @@ class ACL
                     $groupmap[$node->name->__toString()] = $node ;
                 }
             }
+        }
 
         // interpret group privilege data and update user data with group information.
         foreach ($groupmap as $groupkey => $groupNode) {