Fix index message not found error on transport

Author: ffouchier

src/Transports/Transport.php

@@ -250,7 +250,7 @@ protected function arrayGet(array $array, string $key)
         $exploded = explode('.', $key, 2);
 
         if (!isset($exploded[1])) {
-            return $array[$key];
+            return $array[$key] ?? null;
         }
 
         return $this->arrayGet($array[$exploded[0]], $exploded[1]);