Error: Call to a member function bundle() on string in bootstrap_for_drupal_theme_suggestions_page_alter()

Problem/Motivation

Reverting revisions fails with this error message:

The website encountered an unexpected error. Please try again later.
Error: Call to a member function bundle() on string in bootstrap_for_drupal_theme_suggestions_page_alter() (line 60 of themes/contrib/bfd/bootstrap_for_drupal.theme).

bootstrap_for_drupal_theme_suggestions_page_alter() (Line: 449)
Drupal\Core\Theme\ThemeManager->alterForTheme() (Line: 458)
Drupal\Core\Theme\ThemeManager->alter() (Line: 245)
Drupal\Core\Theme\ThemeManager->render() (Line: 436)
Drupal\Core\Render\Renderer->doRender() (Line: 201)
Drupal\Core\Render\Renderer->render() (Line: 450)
Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 85)
__TwigTemplate_53bb03807a561de7c0bb9937cf2b80bfa871734e3ff2597055f2f8d979c514d2->doDisplay() (Line: 405)
Twig\Template->displayWithErrorHandling() (Line: 378)
Twig\Template->display() (Line: 390)
Twig\Template->render() (Line: 65)
twig_render_template() (Line: 384)
Drupal\Core\Theme\ThemeManager->render() (Line: 436)
Drupal\Core\Render\Renderer->doRender() (Line: 201)
Drupal\Core\Render\Renderer->render() (Line: 162)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 578)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 163)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse() (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray()
call_user_func() (Line: 142)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch() (Line: 163)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 57)
Drupal\Core\StackMiddleware\Session->handle() (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 23)
Stack\StackedHttpKernel->handle() (Line: 717)
Drupal\Core\DrupalKernel->handle() (Line: 19)

This issue has the same error: https://www.drupal.org/project/bfd/issues/3193960 →

Steps to reproduce

Use a bfd subtheme, try to revert a node to a previous revision.

I have not tested on a clean install of just bfd. I'm guessing it's not specific to my set up since there is another issue with the same error.

Oh, I did try to run a clean instance via SimplyTest.me, but it didn't build correctly.

Proposed resolution

No idea. When I look at the code, it is not clear what it is actually trying to do, so I'm not going to try and fix it.

Remaining tasks

User interface changes

API changes

Data model changes