fix for return exception

src/Meshcore.php

@@ -140,7 +140,7 @@ class Meshcore {
      * @param bool $async
      * @return null|MessageResponse 
      */
-    public function getNextMessage(bool $async = false): ?MessageResponse {
+    public function getNextMessage(bool $async = false): MessageResponse | NoMoreMessagesResponse {
         CoreProtocol::writeFrame($this->serial, chr(CoreProtocol::CMD_SYNC_NEXT_MESSAGE));
 
         return $this->waitForResponse([MessageResponse::class, NoMoreMessagesResponse::class], $async);