fix notice in SystemComponent, a=chris

diff --git a/src/controllers/components/SystemComponent.php b/src/controllers/components/SystemComponent.php
index a59c3d3a2..a0803877e 100755
--- a/src/controllers/components/SystemComponent.php
+++ b/src/controllers/components/SystemComponent.php
@@ -66,7 +66,7 @@ class SystemComponent extends Component
         $data["ELEMENT"] = "managemachines";
         $possible_arguments = ["addmachine",  "deletemachine", "disablejob",
             "enablejob", "log", "mediajobs", "update", "updatemode"];
-        $data['SCRIPT'] = (!in_array($_REQUEST['arg'],
+        $data['SCRIPT'] = (empty($_REQUEST['arg'])|| !in_array($_REQUEST['arg'],
             ["disablejob", "enablejob", "mediajobs", "updatemode"])) ?
             "doUpdate();" : "";
         $data["leftorright"]=(L\getLocaleDirection() == 'ltr') ? "right":