getAppPath can return false

Fixes https://github.com/owncloud/core/issues/23533

lib/private/route/router.php

@@ -95,9 +95,12 @@ class Router implements IRouter {
 		if (!isset($this->routingFiles)) {
 			$this->routingFiles = [];
 			foreach (\OC_APP::getEnabledApps() as $app) {
-				$file = \OC_App::getAppPath($app) . '/appinfo/routes.php';
-				if (file_exists($file)) {
-					$this->routingFiles[$app] = $file;
+				$appPath = \OC_App::getAppPath($app);
+				if($appPath !== false) {
+					$file = $appPath . '/appinfo/routes.php';
+					if (file_exists($file)) {
+						$this->routingFiles[$app] = $file;
+					}
 				}
 			}
 		}