TypeError carsh when no default_value in form element

a view with a date filter started crashing after activating this module, apparently because the form element doesn't have default values set.

TypeError: Drupal\date_popup\DatetimePopup::setDefaultValue(): Argument #1 ($default_value) must be of type string, null given, called in /srv/www/htdocs/drupal/html/modules/contrib/date_popup/src/DatePopupTrait.php on line 70 in Drupal\date_popup\DatetimePopup::setDefaultValue() (line 22 of modules/contrib/date_popup/src/DatePopupTrait.php).

Drupal\date_popup\DatetimePopup::applyDatePopupToForm() (Line: 20)
Drupal\date_popup\DatetimePopup->buildExposedForm() (Line: 104)
Drupal\views\Form\ViewsExposedForm->buildForm()
call_user_func_array() (Line: 536)
Drupal\Core\Form\FormBuilder->retrieveForm() (Line: 284)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 134)
Drupal\views\Plugin\views\exposed_form\ExposedFormPluginBase->renderExposedForm() (Line: 1298)
Drupal\views\ViewExecutable->build() (Line: 393)
Drupal\views\Plugin\views\display\PathPluginBase->execute() (Line: 198)
Drupal\views\Plugin\views\display\Page->execute() (Line: 1690)
Drupal\views\ViewExecutable->executeDisplay() (Line: 81)
Drupal\views\Element\View::preRenderViewElement()
call_user_func_array() (Line: 113)
Drupal\Core\Render\Renderer->doTrustedCallback() (Line: 870)
Drupal\Core\Render\Renderer->doCallback() (Line: 432)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 238)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 231)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare() (Line: 128)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse() (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray()
call_user_func() (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch() (Line: 186)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 53)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 741)
Drupal\Core\DrupalKernel->handle() (Line: 19)