Solved it by surrounding like this
if (T4::isT4())
{
$path = T4PATH_LOCAL . '/' . $file;
$uri = T4PATH_LOCAL_URI . '/' . $which;
if (is_file($path)) return $getUri ? self::getUri($uri, $relative) : $path;
$path = T4PATH_TPL . '/' . $file;
$uri = T4PATH_TPL_URI . '/' . $which;
if (is_file($path)) return $getUri ? self::getUri($uri, $relative) : $path;
$path = T4PATH_BASE . '/' . $file;
$uri = T4PATH_BASE_URI . '/' . $which;
if (is_file($path)) return $getUri ? self::getUri($uri, $relative) : $path;
}
in /plugins/system/t4/src/t4/Helper/Path.php for findInTheme() and findInBase()