Probably the error occurs when the field to calculate for readtime is empty.
After hiding the readtime field from view mode, there was no error.
Drupal\Core\Database\IntegrityConstraintViolationException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'nid' cannot be null: INSERT INTO {read_time} (nid, read_time) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1); Array ( [:db_insert_placeholder_0] => [:db_insert_placeholder_1] => 0 ) in Drupal\Core\Database\Connection->handleQueryException() (line 689 of /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/lib/Drupal/Core/Database/Connection.php).
Warning: Illegal string offset 'value' in Drupal\views\Plugin\views\area\Text->preQuery() (line 50 of /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/modules/views/src/Plugin/views/area/Text.php) #0 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/includes/bootstrap.inc(587): _drupal_error_handler_real(2, 'Illegal string ...', '/srv/bindings/5...', 50, Array) #1 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/modules/views/src/Plugin/views/area/Text.php(50): _drupal_error_handler(2, 'Illegal string ...', '/srv/bindings/5...', 50, Array) #2 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/modules/views/src/ViewExecutable.php(1011): Drupal\views\Plugin\views\area\Text->preQuery() #3 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/modules/views/src/ViewExecutable.php(1233): Drupal\views\ViewExecutable->_preQuery() #4 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/modules/views/src/Plugin/views/display/PathPluginBase.php(390): Drupal\views\ViewExecutable->build() #5 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/modules/views/src/Plugin/views/display/Page.php(180): Drupal\views\Plugin\views\display\PathPluginBase->execute() #6 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/modules/views/src/ViewExecutable.php(1630): Drupal\views\Plugin\views\display\Page->execute() #7 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/modules/views/src/Element/View.php(77): Drupal\views\ViewExecutable->executeDisplay('page', Array) #8 [internal function]: Drupal\views\Element\View::preRenderViewElement(Array) #9 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/lib/Drupal/Core/Render/Renderer.php(378): call_user_func(Array, Array) #10 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/lib/Drupal/Core/Render/Renderer.php(195): Drupal\Core\Render\Renderer->doRender(Array, false) #11 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(226): Drupal\Core\Render\Renderer->render(Array, false) #12 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/lib/Drupal/Core/Render/Renderer.php(582): Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() #13 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(227): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure)) #14 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/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)) #15 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/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)) #16 [internal function]: Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object(Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent), 'kernel.view', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher)) #17 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/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)) #18 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/vendor/symfony/http-kernel/HttpKernel.php(156): Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object(Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent)) #19 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/vendor/symfony/http-kernel/HttpKernel.php(68): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1) #20 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/lib/Drupal/Core/StackMiddleware/Session.php(57): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #21 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #22 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #23 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true) #24 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #25 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(52): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #26 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #27 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/core/lib/Drupal/Core/DrupalKernel.php(693): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #28 /srv/bindings/551d86133a374fbd8e5ab32def27ab47/code/web/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request)) #29 {main}.
Thank you.