Silence a "undefined index" error

Bug 1402912

Change-Id: I519a7ef9bb116baf6fe1186d2e5de681a1543e77

htdocs/blocktype/text/lib.php

@@ -29,8 +29,12 @@ class PluginBlocktypeText extends SystemBlocktype {
         safe_require('artefact', 'file');
         $configdata = $instance->get('configdata');
         // Add all artefacts found in the text
-        $text = $configdata['text'];
-        $artefacts = array_unique(artefact_get_references_in_html($text));
+        if (empty($configdata['text'])) {
+            $artefacts = array();
+        }
+        else {
+            $artefacts = array_unique(artefact_get_references_in_html($configdata['text']));
+        }
         return $artefacts;
     }