Handle "Response has no signature" exception

[BaluErtl]

In certain conditions (eg. on localhost and/or with insufficient API key configuration) the following exception is being thrown:

A webhelyen nem várt hiba történt. Érdemes később újrapróbálkozni.

Exception: Response has no signature in Cheppers\OtpspClient\OtpSimplePayClient->getMessageBody() (line 326 of /Applications/MAMP/htdocs/elter.hu/vendor/cheppers/otpsp-client/src/OtpSimplePayClient.php).
Cheppers\OtpspClient\OtpSimplePayClient->startPayment(Object) (Line: 55)

Drupal\commerce_otpsp\PluginForm\SimplePayRedirectOffSitePaymentForm->buildConfigurationForm(Array, Object) (Line: 108)
Drupal\commerce_payment\Plugin\Commerce\InlineForm\PaymentGatewayForm->buildInlineForm(Array, Object) (Line: 218)
Drupal\commerce_payment\Plugin\Commerce\CheckoutPane\PaymentProcess->buildPaneForm(Array, Object, Array) (Line: 559)
Drupal\commerce_checkout\Plugin\Commerce\CheckoutFlow\CheckoutFlowWithPanesBase->buildForm(Array, Object, 'payment')
call_user_func_array(Array, Array) (Line: 532)
Drupal\Core\Form\FormBuilder->retrieveForm('commerce_checkout_flow_multistep_default', Object) (Line: 278)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 219)
Drupal\Core\Form\FormBuilder->getForm(Object, 'payment') (Line: 94)
Drupal\commerce_checkout\Controller\CheckoutController->formPage(Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 573)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 708)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

As it's visible to the end-user (webshop customer), would be nice to handle properly.