Warning: foreach() argument must be of type array|object, null given

Problem/Motivation

Every so often, I see this error in my watchdog log:

Warning: foreach() argument must be of type array|object, null given in Drupal\visitors\Controller\Visitors->doCounter() (line 311 of /var/www/mysite/web/modules/contrib/visitors/src/Controller/Visitors.php)
#0 /var/www/mysite/web/core/includes/bootstrap.inc(104): _drupal_error_handler_real()
#1 /var/www/mysite/web/modules/contrib/visitors/src/Controller/Visitors.php(311): _drupal_error_handler()
#2 /var/www/mysite/web/modules/contrib/visitors/src/Controller/Visitors.php(185): Drupal\visitors\Controller\Visitors->doCounter()
#3 [internal function]: Drupal\visitors\Controller\Visitors->track()
#4 /var/www/mysite/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array()
#5 /var/www/mysite/web/core/lib/Drupal/Core/Render/Renderer.php(627): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#6 /var/www/mysite/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(121): Drupal\Core\Render\Renderer->executeInRenderContext()
#7 /var/www/mysite/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext()
#8 /var/www/mysite/vendor/symfony/http-kernel/HttpKernel.php(183): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#9 /var/www/mysite/vendor/symfony/http-kernel/HttpKernel.php(76): Symfony\Component\HttpKernel\HttpKernel->handleRaw()
#10 /var/www/mysite/web/core/lib/Drupal/Core/StackMiddleware/Session.php(53): Symfony\Component\HttpKernel\HttpKernel->handle()
#11 /var/www/mysite/web/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\Core\StackMiddleware\Session->handle()
#12 /var/www/mysite/web/core/lib/Drupal/Core/StackMiddleware/ContentLength.php(28): Drupal\Core\StackMiddleware\KernelPreHandle->handle()
#13 /var/www/mysite/web/core/modules/big_pipe/src/StackMiddleware/ContentLength.php(32): Drupal\Core\StackMiddleware\ContentLength->handle()
#14 /var/www/mysite/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(116): Drupal\big_pipe\StackMiddleware\ContentLength->handle()
#15 /var/www/mysite/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(90): Drupal\page_cache\StackMiddleware\PageCache->pass()
#16 /var/www/mysite/web/modules/contrib/cleantalk/src/EventSubscriber/BootSubscriber.php(206): Drupal\page_cache\StackMiddleware\PageCache->handle()
#17 /var/www/mysite/web/core/modules/ban/src/BanMiddleware.php(50): Drupal\cleantalk\EventSubscriber\BootSubscriber->handle()
#18 /var/www/mysite/web/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Drupal\ban\BanMiddleware->handle()
#19 /var/www/mysite/web/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle()
#20 /var/www/mysite/web/core/lib/Drupal/Core/StackMiddleware/AjaxPageState.php(53): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle()
#21 /var/www/mysite/web/core/lib/Drupal/Core/StackMiddleware/StackedHttpKernel.php(51): Drupal\Core\StackMiddleware\AjaxPageState->handle()
#22 /var/www/mysite/web/core/lib/Drupal/Core/DrupalKernel.php(715): Drupal\Core\StackMiddleware\StackedHttpKernel->handle()
#23 /var/www/mysite/web/index.php(19): Drupal\Core\DrupalKernel->handle()
#24 {main}

Steps to reproduce

I don't know how to reproduce this.

Proposed resolution

The error is caused because the PHP json_decode function can return null, and the code isn't checking for that condition:

    $viewed = NULL;
    if (!is_null($cvar)) {
      $custom = json_decode($cvar);
      foreach ($custom as $c) {
        if ($c[0] == 'viewed') {
          $viewed = $c[1];
        }
      }
    }

The fix here is to check the value of $custom that's returned from json_decode() and only enter the foreach loop if it isn't null.