Registration fails due to bad json input

Problem/Motivation

We've now seen a couple of failed registrations with this stack trace:

    "#0 /var/www/html/web/modules/contrib/pf_onesignal/src/Controller/Register.php(90): json_decode()",
    "#1 [internal function]: Drupal\\\\pf_onesignal\\\\Controller\\\\Register->execute()",
    "#2 /var/www/html/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array()",
    "#3 /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php(580): Drupal\\\\Core\\\\EventSubscriber\\\\EarlyRenderingControllerWrapperSubscriber->Drupal\\\\Core\\\\EventSubscriber\\\\{closure}()",
    "#4 /var/www/html/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(124): Drupal\\\\Core\\\\Render\\\\Renderer->executeInRenderContext()",
    "#5 /var/www/html/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\\\\Core\\\\EventSubscriber\\\\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext()",
    "#6 /var/www/html/vendor/symfony/http-kernel/HttpKernel.php(169): Drupal\\\\Core\\\\EventSubscriber\\\\EarlyRenderingControllerWrapperSubscriber->Drupal\\\\Core\\\\EventSubscriber\\\\{closure}()",
    "#7 /var/www/html/vendor/symfony/http-kernel/HttpKernel.php(81): Symfony\\\\Component\\\\HttpKernel\\\\HttpKernel->handleRaw()",
    "#8 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/Session.php(58): Symfony\\\\Component\\\\HttpKernel\\\\HttpKernel->handle()",
    "#9 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\\\\Core\\\\StackMiddleware\\\\Session->handle()",
    "#10 /var/www/html/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\\\\Core\\\\StackMiddleware\\\\KernelPreHandle->handle()",
    "#11 /var/www/html/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\\\\page_cache\\\\StackMiddleware\\\\PageCache->pass()",
    "#12 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Drupal\\\\page_cache\\\\StackMiddleware\\\\PageCache->handle()",
    "#13 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\\\\Core\\\\StackMiddleware\\\\ReverseProxyMiddleware->handle()",
    "#14 /var/www/html/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\\\\Core\\\\StackMiddleware\\\\NegotiationMiddleware->handle()",
    "#15 /var/www/html/web/core/lib/Drupal/Core/DrupalKernel.php(707): Stack\\\\StackedHttpKernel->handle()",
    "#16 /var/www/html/web/index.php(19): Drupal\\\\Core\\\\DrupalKernel->handle()",
    "#17 {main}"

Proposed resolution

Improve logging so that we can find out more details.