Warning: Undefined property: MultistepDefault::$order in order_information step as anonymous user

fixed the issue and created MR, please review it once

I have the same issue. Appears only in logs e.g. when changing the payment method
Location: checkout/xxx/order_information?_wrapper_format=drupal_ajax&ajax_form=1
- Drupal 10.3.0
- Commerce 8.x-2.40

Same here on Drupal 10.4.3 Commerce 8.x-2.4. The warning appears on the page, for logged-in and anonymous users:

Warning: Undefined property: Drupal\commerce_checkout\Plugin\Commerce\CheckoutFlow\MultistepDefault::$order in Drupal\commerce_checkout\Plugin\Commerce\CheckoutFlow\CheckoutFlowBase->getOrder() (line 167 of modules/contrib/commerce/modules/checkout/src/Plugin/Commerce/CheckoutFlow/CheckoutFlowBase.php).
Drupal\commerce_checkout\Plugin\Commerce\CheckoutFlow\CheckoutFlowBase->getOrder() (Line: 233)
Drupal\commerce_checkout\Plugin\Commerce\CheckoutFlow\CheckoutFlowBase->getStepId() (Line: 50)
Drupal\commerce_checkout\CheckoutOrderManager->getCheckoutStepId() (Line: 94)
Drupal\commerce_checkout\Plugin\Block\CheckoutProgressBlock->build() (Line: 171)
Drupal\block\BlockViewBuilder::preRender()
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: 504)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 484)
Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 100)
__TwigTemplate_130ea0b20e16b7458dc8789110f80010->doDisplay() (Line: 388)
Twig\Template->yield() (Line: 344)
Twig\Template->display() (Line: 359)
Twig\Template->render() (Line: 51)
Twig\TemplateWrapper->render() (Line: 33)
twig_render_template() (Line: 348)
Drupal\Core\Theme\ThemeManager->render() (Line: 491)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 484)
Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 91)
__TwigTemplate_80e4a832c064768b4b184dd17715a366->doDisplay() (Line: 388)
Twig\Template->yield() (Line: 344)
Twig\Template->display() (Line: 359)
Twig\Template->render() (Line: 51)
Twig\TemplateWrapper->render() (Line: 33)
twig_render_template() (Line: 348)
Drupal\Core\Theme\ThemeManager->render() (Line: 491)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 158)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 153)
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: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle() (Line: 116)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 90)
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)

I merged in the latest changes from 3.x and it turns out the main cause of the issue was fixed recently, in #3535420: Fix phpstan and stylelint issues.

The current patch makes this a bit more robust, but probably the bug is already fixed in the latest 3.x-dev.

I'm willing to commit this but there are currently merge conflicts.
@pfrenssen: Would you be able to solve this please?

Sure! Done :)

Thank you!