Fatal error when attempting to access node revisions

I am encountering a fatal error when attempting to access the revision tab of any node (e.g. example.com/node/30322/revisions) after the latest update of this theme (note this is 2.1.10, and not 2.1.9 as selected, as the new version is not currently available to select here).

Here's the watchdog message:

Error: [] operator not supported for strings in bootstrap4_preprocess_table() (line 495 of /public_html/themes/contrib/bootstrap4/bootstrap4.theme) #0 /public_html/core/lib/Drupal/Core/Theme/ThemeManager.php(287): bootstrap4_preprocess_table(Array, 'table', Array)
#1 /public_html/core/lib/Drupal/Core/Render/Renderer.php(431): Drupal\Core\Theme\ThemeManager->render('table', Array)
#2 /public_html/core/lib/Drupal/Core/Render/Renderer.php(444): Drupal\Core\Render\Renderer->doRender(Array)
#3 /public_html/core/lib/Drupal/Core/Render/Renderer.php(200): Drupal\Core\Render\Renderer->doRender(Array, false)
#4 /public_html/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(226): Drupal\Core\Render\Renderer->render(Array, false)
#5 /public_html/core/lib/Drupal/Core/Render/Renderer.php(573): Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}()
#6 /public_html/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(227): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure))
#7 /public_html/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(117): Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\CurrentRouteMatch))
#8 /public_html/core/lib/Drupal/Core/EventSubscriber/MainContentViewSubscriber.php(90): Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\CurrentRouteMatch))
#9 [internal function]: Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object(Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent), 'kernel.view', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher))
#10 /public_html/core/lib/Drupal/Component/EventDispatcher/ContainerAwareEventDispatcher.php(111): call_user_func(Array, Object(Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent), 'kernel.view', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher))
#11 /vendor/symfony/http-kernel/HttpKernel.php(156): Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object(Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent))
#12 /vendor/symfony/http-kernel/HttpKernel.php(68): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1)
#13 /public_html/core/lib/Drupal/Core/StackMiddleware/Session.php(57): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#14 /public_html/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#15 /public_html/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#16 /public_html/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#17 /public_html/modules/contrib/cleantalk/src/EventSubscriber/BootSubscriber.php(287): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#18 /public_html/core/modules/ban/src/BanMiddleware.php(50): Drupal\cleantalk\EventSubscriber\BootSubscriber->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#19 /public_html/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\ban\BanMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#20 /public_html/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(52): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#21 /vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#22 /public_html/core/lib/Drupal/Core/DrupalKernel.php(708): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#23 /public_html/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request))
#24 {main} .