fix deprecated access to GET variable

index.php

@@ -76,8 +76,10 @@ if ( ! is_array($languages) ) {
     $languages = [$languages];
 }
 $f3->set('default_lang', array_shift($languages));
-if (in_array(strtolower($f3->get('GET.lang')), $languages)) {
-    $f3->set('LANG', strtolower($f3->get('GET.lang')));
+
+$getLang = null !== $f3->get('GET.lang') ? strtolower($f3->get('GET.lang')) : false;
+if (isset($getLang) && in_array($getLang, $languages)) {
+    $f3->set('LANG', $getLang);
 } else {
     $f3->set('LANG', $f3->get('default_lang'));
 }