batch_set function expects Countable|array but gets null.

Problem/Motivation

While testing version 1.15 of this module I'm getting this error message:

The website encountered an unexpected error. Please try again later.
TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in batch_set() (line 777 of core/includes/form.inc).

It appears that the issue is with a form submission and the processing of a CSV file that is being uploaded as part of that form. Specifically, the error message is indicating that a null value is being passed to the batch_set() function, which is causing a "TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given" error. This suggests that the variable that is being passed to batch_set() is not being set properly, and is instead coming in as null.

It is possible that the problem is with the form submission and the way that the CSV file is being passed to the PHP file, or it may be an issue with the code that processes the CSV file data within the ImporterBase->process() method.

Steps to reproduce

Using Drupal version 9.4.8 and PHP 8.1.14 with module version 1.15. Imported a test .csv file that previously worked with no issues.

Here is the full error message

TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in batch_set() (line 777 of /code/core/includes/form.inc)
#0 /code/modules/csv_importer/src/Plugin/ImporterBase.php(232): batch_set(Array)
#1 /code/modules/csv_importer/src/Form/ImporterForm.php(368): Drupal\csv_importer\Plugin\ImporterBase->process()
#2 [internal function]: Drupal\csv_importer\Form\ImporterForm->submitForm(Array, Object(Drupal\Core\Form\FormState))
#3 /code/core/lib/Drupal/Core/Form/FormSubmitter.php(114): call_user_func_array(Array, Array)
#4 /code/core/lib/Drupal/Core/Form/FormSubmitter.php(52): Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object(Drupal\Core\Form\FormState))
#5 /code/core/lib/Drupal/Core/Form/FormBuilder.php(592): Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object(Drupal\Core\Form\FormState))
#6 /code/core/lib/Drupal/Core/Form/FormBuilder.php(320): Drupal\Core\Form\FormBuilder->processForm('csv_importer_fo...', Array, Object(Drupal\Core\Form\FormState))
#7 /code/core/lib/Drupal/Core/Controller/FormController.php(73): Drupal\Core\Form\FormBuilder->buildForm(Object(Drupal\csv_importer\Form\ImporterForm), Object(Drupal\Core\Form\FormState))
#8 [internal function]: Drupal\Core\Controller\FormController->getContentResult(Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\RouteMatch))
#9 /code/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array(Array, Array)
#10 /code/core/lib/Drupal/Core/Render/Renderer.php(564): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#11 /code/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(124): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure))
#12 /code/modules/jsonapi_earlyrendering_workaround/src/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(31): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
#13 /code/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\jsonapi_earlyrendering_workaround\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
#14 /code/vendor/symfony/http-kernel/HttpKernel.php(169): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#15 /code/vendor/symfony/http-kernel/HttpKernel.php(81): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1)
#16 /code/modules/contrib/simple_oauth/src/HttpMiddleware/BasicAuthSwap.php(68): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#17 /code/core/lib/Drupal/Core/StackMiddleware/Session.php(58): Drupal\simple_oauth\HttpMiddleware\BasicAuthSwap->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#18 /code/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#19 /code/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#20 /code/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#21 /code/vendor/asm89/stack-cors/src/Asm89/Stack/Cors.php(49): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#22 /code/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Asm89\Stack\Cors->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#23 /code/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#24 /code/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#25 /code/core/lib/Drupal/Core/DrupalKernel.php(709): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#26 /code/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request))
#27 {main}