D8 port: PHP warnings from config page

I've tracked this down to the "site map page" link in the description for the page title field. Based on the error and stack trace (below), investigation should center around StringTranslationTrait::t

Warning: htmlspecialchars() expects parameter 1 to be string, object given in Drupal\Component\Utility\Html::escape() (line 407 of /Users/Swan/Sites/locals/d8-sandbox.local/core/lib/Drupal/Component/Utility/Html.php).

Drupal\Component\Utility\Html::escape(Object)
Drupal\Component\Render\FormattableMarkup::placeholderEscape(Object)
Drupal\Component\Render\FormattableMarkup::placeholderFormat('Page title that will be used on the <a href="@link">site map page</a>.', Array)
Drupal\Core\StringTranslation\TranslatableMarkup->render()
Drupal\Core\StringTranslation\TranslatableMarkup->__toString()
__TwigTemplate_462daf6ce6e4590b050e88b89c6a67976575dfa37e63df18b5f911e9e1ab9361->doDisplay(Array, Array)
Twig_Template->displayWithErrorHandling(Array, Array)
Twig_Template->display(Array)
Twig_Template->render(Array)
twig_render_template('core/themes/classy/templates/form/form-element.html.twig', Array)
Drupal\Core\Theme\ThemeManager->render('form_element', Array)
Drupal\Core\Render\Renderer->doRender(Array)
Drupal\Core\Render\Renderer->doRender(Array, )
Drupal\Core\Render\Renderer->render(Array)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1)
__TwigTemplate_811942a2def36eadacf89dc2697df7b349fabe6eff64dbb5f9bfac1189008b18->doDisplay(Array, Array)
Twig_Template->displayWithErrorHandling(Array, Array)
Twig_Template->display(Array)
Twig_Template->render(Array)
twig_render_template('core/modules/system/templates/system-config-form.html.twig', Array)
Drupal\Core\Theme\ThemeManager->render('system_config_form', Array)
Drupal\Core\Render\Renderer->doRender(Array, )
Drupal\Core\Render\Renderer->render(Array, )
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}()
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object, Object)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)