Deprecated function: in_array(): Passing null to parameter #3 ($strict) of type bool is deprecated in Drupal\fillpdf\Plugin\PdfBackend\PdftkPdfBackend->parseFile() (line 145

Problem/Motivation

D9.5.1 on php 8.1 using pdftk (2.02-5) from ubuntu 22.04
gives multiple errors like this:

Error message

    Deprecated function: in_array(): Passing null to parameter #3 ($strict) of type bool is deprecated in Drupal\fillpdf\Plugin\PdfBackend\PdftkPdfBackend->parseFile() (line 145 of modules/contrib/fillpdf/src/Plugin/PdfBackend/PdftkPdfBackend.php).

    Drupal\fillpdf\Plugin\PdfBackend\PdftkPdfBackend->parseFile(Object) (Line: 102)
    Drupal\fillpdf\InputHelper->parseFields(Object) (Line: 79)
    Drupal\fillpdf\InputHelper->attachPdfToForm(Object) (Line: 177)
    Drupal\fillpdf\Form\FillPdfOverviewForm->submitForm(Array, Object)
    call_user_func_array(Array, Array) (Line: 114)
    Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 52)
    Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 595)
    Drupal\Core\Form\FormBuilder->processForm('fillpdf_forms_admin', Array, Object) (Line: 323)
    Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
    Drupal\Core\Controller\FormController->getContentResult(Object, Object)
    call_user_func_array(Array, Array) (Line: 123)
    Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 580)
    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: 169)
    Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 81)
    Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
    Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
    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: 48)
    Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
    Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
    Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 713)
    Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes