Fatal error when inserting Amazon keys

Hi,
when inserting the Amazon keys and saving I always receive a fatal error. This i the log that WordPress sends me via email. Thanks!

Un errore di E_ERROR è stato causato nella linea 16 del file 
root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/RejectedPromise.php. 

Messaggio di errore: 
Uncaught TypeError: method_exists(): Argument #1 ($object_or_class) must be of type object|string, array given in root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/RejectedPromise.php:16

Stack trace:

0 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/RejectedPromise.php(16): method_exists()

1 root/wp-content/plugins/mailster-amazonses/vendor/Aws/Handler/GuzzleV6/GuzzleHandler.php(41): Mailster\Aws3\GuzzleHttp\Promise\RejectedPromise->__construct()

2 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/Promise.php(164): Mailster\Aws3\Aws\Handler\GuzzleV6\GuzzleHandler::Mailster\Aws3\Aws\Handler\GuzzleV6{closure}()

3 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/Promise.php(125): Mailster\Aws3\GuzzleHttp\Promise\Promise::callHandler()

4 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/TaskQueue.php(44): Mailster\Aws3\GuzzleHttp\Promise\Promise::Mailster\Aws3\GuzzleHttp\Promise{closure}()

5 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Handler/CurlMultiHandler.php(80): Mailster\Aws3\GuzzleHttp\Promise\TaskQueue->run()

6 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Handler/CurlMultiHandler.php(101): Mailster\Aws3\GuzzleHttp\Handler\CurlMultiHandler->tick()

7 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/Promise.php(200): Mailster\Aws3\GuzzleHttp\Handler\CurlMultiHandler->execute()

8 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/Promise.php(183): Mailster\Aws3\GuzzleHttp\Promise\Promise->invokeWaitFn()

9 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/Promise.php(219): Mailster\Aws3\GuzzleHttp\Promise\Promise->waitIfPending()

10 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/Promise.php(185): Mailster\Aws3\GuzzleHttp\Promise\Promise->invokeWaitList()

11 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/Promise.php(219): Mailster\Aws3\GuzzleHttp\Promise\Promise->waitIfPending()

12 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/Promise.php(185): Mailster\Aws3\GuzzleHttp\Promise\Promise->invokeWaitList()

13 root/wp-content/plugins/mailster-amazonses/vendor/GuzzleHttp/Promise/Promise.php(51): Mailster\Aws3\GuzzleHttp\Promise\Promise->waitIfPending()

14 root/wp-content/plugins/mailster-amazonses/vendor/Aws/AwsClientTrait.php(43): Mailster\Aws3\GuzzleHttp\Promise\Promise->wait()

15 root/wp-content/plugins/mailster-amazonses/vendor/Aws/AwsClientTrait.php(56): Mailster\Aws3\Aws\AwsClient->execute()

16 root/wp-content/plugins/mailster-amazonses/classes/amazonses.class.php(192): Mailster\Aws3\Aws\AwsClient->__call()

17 root/wp-content/plugins/mailster-amazonses/classes/amazonses.class.php(206): MailsterAmazonSES->aws()

18 root/wp-content/plugins/mailster-amazonses/classes/amazonses.class.php(306): MailsterAmazonSES->getquota()

19 root/wp-includes/class-wp-hook.php(308): MailsterAmazonSES->verify_options()

20 root/wp-includes/plugin.php(205): WP_Hook->apply_filters()

21 root/wp-content/plugins/mailster/classes/settings.class.php(1271): apply_filters()

22 root/wp-includes/class-wp-hook.php(310): MailsterSettings->verify()

23 root/wp-includes/plugin.php(205): WP_Hook->apply_filters()

24 root/wp-includes/formatting.php(5000): apply_filters()

25 root/wp-includes/option.php(448): sanitize_option()

26 root/wp-admin/options.php(322): update_option()

27 {main}

thrown