FullPageVariant::$mainContent must always be an array

Problem/Motivation

When creating a page with a "Full Page Variant" I get the following fatal error:

Recoverable fatal error: Argument 1 passed to Drupal\system\Plugin\Block\SystemMainBlock::setMainContent() must be of the type array, null given, called in core/modules/block/src/Plugin/DisplayVariant/FullPageVariant.php on line 111 and defined in Drupal\system\Plugin\Block\SystemMainBlock->setMainContent() (line 34 of core/modules/system/src/Plugin/Block/SystemMainBlock.php).

Drupal\system\Plugin\Block\SystemMainBlock->setMainContent(NULL)
Drupal\block\Plugin\DisplayVariant\FullPageVariant->build()
Drupal\page_manager\Entity\PageViewBuilder->view(Object, 'full', NULL)
Drupal\Core\Entity\Controller\EntityViewController->view(Object, 'full', NULL)
call_user_func_array(Array, Array)
Drupal\Core\Controller\HtmlPageController->getContentResult(Object, '\Drupal\Core\Entity\Controller\EntityViewController::view')
Drupal\Core\Controller\HtmlPageController->content(Object, '\Drupal\Core\Entity\Controller\EntityViewController::view')
call_user_func_array(Array, Array)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)

The code was introduced in: https://www.drupal.org/node/2363025

Steps to Reproduce

1. Go to http://localhost/d8/admin/structure/page_manager and add a new page with any path
2. Add new display variant of type "Full Page".
3. Visit page and see fatal error.

Proposed resolution

?

Remaining tasks

Find out what the actual problem is.

User interface changes

n/a

API changes

n/a