CC-5709: Airtime Analyzer

* Some cleanup and refactoring for file deletion in the Media REST API
* Fix the refresh timer on the Add Media page when there's pending
  imports.

airtime_mvc/application/models/MusicDir.php

@@ -301,7 +301,9 @@ SQL;
     public static function getDirByPK($pk)
     {
         $dir = CcMusicDirsQuery::create()->findPK($pk);
-
+        if (!$dir) {
+            return null;
+        }
         $mus_dir = new Application_Model_MusicDir($dir);
 
         return $mus_dir;