Prevent PHP warnings when language undefined (probably due to a missi…

…ng prototype).

renderer.php

@@ -575,7 +575,7 @@ public function correct_response(question_attempt $qa) {
             }
         }
 
-        $uclang = ucwords($currentlanguage);
+        $uclang = ucwords($currentlanguage ?? '');  // Current language can be null when prototype missing.
         $heading = get_string('asolutionis', 'qtype_coderunner');
         $heading = substr($heading, 0, strlen($heading) - 1) . ' (' . $uclang . ')';
         $divid = 'id_div_' . $fieldname;