Site crashes when "Rebuild on every page load" is selected

I'm getting this whenever I attempt to view public-facing pages and "Rebuild the theme registry on every page load" is selected at admin/config/development/devel.

I am using a sub-theme of the Bootstrap theme.

The website encountered an unexpected error. Please try again later.
Error: Class 'Drupal\bootstrap\Bootstrap' not found in include_once() (line 30 of themes/bootstrap/bootstrap.theme).
include_once() (Line: 140)
Drupal\Core\Extension\Extension->load() (Line: 35)
twig_init(Object)
call_user_func('twig_init', Object) (Line: 137)
Drupal\Core\Theme\ThemeInitialization->loadActiveTheme(Object) (Line: 67)
Drupal\Core\Theme\ThemeInitialization->initTheme('tonevoice') (Line: 406)
Drupal\Core\Theme\ThemeManager->initTheme(Object) (Line: 96)
Drupal\Core\Theme\ThemeManager->getActiveTheme() (Line: 206)
Drupal\Core\Theme\Registry->init(NULL) (Line: 250)
Drupal\Core\Theme\Registry->getRuntime() (Line: 175)
Drupal\Core\Entity\EntityViewBuilder->getBuildDefaults(Object, 'full') (Line: 58)
Drupal\node\NodeViewBuilder->getBuildDefaults(Object, 'full') (Line: 138)
Drupal\Core\Entity\EntityViewBuilder->viewMultiple(Array, 'full', NULL) (Line: 111)
Drupal\Core\Entity\EntityViewBuilder->view(Object, 'full') (Line: 96)
Drupal\Core\Entity\Controller\EntityViewController->view(Object, 'full', NULL) (Line: 55)
Drupal\node\Controller\NodeViewController->view(Object, 'full', NULL)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 574)
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}()
call_user_func_array(Object, Array) (Line: 144)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 64)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 656)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)