Make middleware only acting on numeric page uids

public/typo3conf/ext/ep_theme/Classes/Middleware/ReferrerMiddleware.php

@@ -22,6 +22,10 @@ class ReferrerMiddleware implements MiddlewareInterface
 
         $ref = $queryParams['ref'];
 
+        if (false === is_numeric($ref)) {
+            return $handler->handle($request);
+        }
+
         /** @var SiteFinder $siteFinder */
         $siteFinder = GeneralUtility::makeInstance(SiteFinder::class);