Error Adding a Chat room: Notice: Undefined index: weight in Drupal\Core\Entity\Entity\EntityFormDisplay

When creating a chatroom, an error is logged to the registry. It is not display on screen and the chat room is created nonetheless, and it works for authenticated users.
On a clean install, I get this error. Everything else works, except anonymous screens not being automatically updated (see separate issue for that: https://www.drupal.org/node/2776199#comment-11779606 )

This is the error from /admin/reports/dblog:

Location	http://www.xxxxxx.com/chatroom/add
Referrer	http://www.xxxxxx.com/chatroom/add
Message	Notice: Undefined index: weight in Drupal\Core\Entity\Entity\EntityFormDisplay->processForm() (line 200 of /var/www/vhosts/drupal/core/lib/Drupal/Core/Entity/Entity/EntityFormDisplay.php) #0 /var/www/vhosts/drupal/core/includes/bootstrap.inc(548): _drupal_error_handler_real(8, 'Undefined index...', '/var/www/vhosts...', 200, Array) #1 /var/www/vhosts/drupal/core/lib/Drupal/Core/Entity/Entity/EntityFormDisplay.php(200): _drupal_error_handler(8, 'Undefined index...', '/var/www/vhosts...', 200, Array) #2 [internal function]: Drupal\Core\Entity\Entity\EntityFormDisplay->processForm(Array, Object(Drupal\Core\Form\FormState), Array) #3 /var/www/vhosts/drupal/core/lib/Drupal/Core/Form/FormBuilder.php(981): call_user_func_array(Array, Array) #4 /var/www/vhosts/drupal/core/lib/Drupal/Core/Form/FormBuilder.php(557): Drupal\Core\Form\FormBuilder->doBuildForm('chatroom_add_fo...', Array, Object(Drupal\Core\Form\FormState)) #5 /var/www/vhosts/drupal/core/lib/Drupal/Core/Form/FormBuilder.php(314): Drupal\Core\Form\FormBuilder->processForm('chatroom_add_fo...', Array, Object(Drupal\Core\Form\FormState)) #6 /var/www/vhosts/drupal/core/lib/Drupal/Core/Controller/FormController.php(74): Drupal\Core\Form\FormBuilder->buildForm('chatroom_add_fo...', Object(Drupal\Core\Form\FormState)) #7 [internal function]: Drupal\Core\Controller\FormController->getContentResult(Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\RouteMatch)) #8 /var/www/vhosts/drupal/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array(Array, Array) #9 /var/www/vhosts/drupal/core/lib/Drupal/Core/Render/Renderer.php(574): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() #10 /var/www/vhosts/drupal/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(124): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure)) #11 /var/www/vhosts/drupal/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) #12 [internal function]: Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() #13 /var/www/vhosts/drupal/vendor/symfony/http-kernel/HttpKernel.php(139): call_user_func_array(Object(Closure), Array) #14 /var/www/vhosts/drupal/vendor/symfony/http-kernel/HttpKernel.php(62): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1) #15 /var/www/vhosts/drupal/core/lib/Drupal/Core/StackMiddleware/Session.php(57): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #16 /var/www/vhosts/drupal/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #17 /var/www/vhosts/drupal/core/modules/page_cache/src/StackMiddleware/PageCache.php(99): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #18 /var/www/vhosts/drupal/core/modules/page_cache/src/StackMiddleware/PageCache.php(78): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true) #19 /var/www/vhosts/drupal/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #20 /var/www/vhosts/drupal/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(50): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #21 /var/www/vhosts/drupal/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #22 /var/www/vhosts/drupal/core/lib/Drupal/Core/DrupalKernel.php(652): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #23 /var/www/vhosts/drupal/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request)) #24 {main}.