Otherwise a fatal PHP error is thrown since the filesystem is not setup:
```
( ! ) Fatal error: Call to a member function getOwner() on null in /Users/lreschke/Programming/core/lib/private/files/filesystem.php on line 817
Call Stack
1 0.0004 247792 {main}( ) ../index.php:0
2 0.0946
5088776 OC::handleRequest( ) ../index.php:28
3 0.1423
7491280 OC\Route\Router->match( ) ../base.php:748
4 0.1664
8592152 call_user_func:{/Users/lreschke/Programming/core/lib/private/route/router.php:250} ( ) ../router.php:250
5 0.1664
8592216 __lambda_func( ) ../router.php:250
6 0.1665
8599064 require_once( '/Users/lreschke/Programming/core/apps/files_versions/ajax/getVersions.php' ) ../route.php(135) : runtime-created function:1
7 0.1801
9061096 OCA\Files_Versions\Storage::getUidAndFilename( ) ../getVersions.php:7
8 0.1801
9061144 OC\Files\Filesystem::getOwner( ) ../storage.php:45
```
Furthermore this adds the CSRF check as a nice-to-have addition, the requests accessing this are done via the jQuery AJAX methods and thus the header is automatically added to all requests already.
<?php
+OCP\JSON::checkLoggedIn();
+OCP\JSON::callCheck();
OCP\JSON::checkAppEnabled('files_versions');
$source = $_GET['source'];
<?php
+OCP\JSON::checkLoggedIn();
OCP\JSON::checkAppEnabled('files_versions');
OCP\JSON::callCheck();