Unsupported operand types in rdf module

Hi.

There seems to be some sort of incompatibility issue with Monster Menus and the RDF module. Extract from the error message:

[error] 5#5: *177 FastCGI sent in stderr: "PHP message: Error: Unsupported operand types in /var/www/html/core/modules/rdf/rdf.module on line 219

Steps to reproduce the issue:

1. Set up a fresh Drupal 8 installation
2. Add an article
3. Install Monters Menus: composer require drupal/monster_menus:1.x-dev
4. Add another article
5. Enable the Monster Menus Module
6. Trying to add another article (/mm/7/contents/add) result in this error:
   

   drupal_fpm | [22-Mar-2018 13:51:32] WARNING: [pool www] child 7 said into stderr: "NOTICE: PHP message: Error: Unsupported operand types in /var/www/html/core/modules/rdf/rdf.module on line 219 #0 [internal function]: rdf_entity_prepare_view('node', Array, Array, 'full')"
   drupal_nginx | 2018/03/22 13:51:32 [error] 5#5: *177 FastCGI sent in stderr: "PHP message: Error: Unsupported operand types in /var/www/html/core/modules/rdf/rdf.module on line 219 #0 [internal function]: rdf_entity_prepare_view('node', Array, Array, 'full')
   drupal_nginx | #1 /var/www/html/core/lib/Drupal/Core/Extension/ModuleHandler.php(402): call_user_func_array('rdf_entity_prep...', Array)
   drupal_nginx | #2 /var/www/html/core/lib/Drupal/Core/Entity/EntityViewBuilder.php(317): Drupal\Core\Extension\ModuleHandler->invokeAll('entity_prepare_...', Array)
   drupal_nginx | #3 /var/www/html/core/modules/node/src/NodeViewBuilder.php(22): Drupal\Core\Entity\EntityViewBuilder->buildComponents(Array, Array, Array, 'full')
   drupal_nginx | #4 /var/www/html/core/lib/Drupal/Core/Entity/EntityViewBuilder.php(263): Drupal\node\NodeViewBuilder->buildComponents(Array, Array, Array, 'full')
   drupal_nginx | #5 /var/www/html/core/lib/Drupal/Core/Entity/EntityViewBuilder.php(220): Drupal\Core\Entity\EntityViewBuilder->buildMultiple(Array)
   drupal_nginx | #6 [internal function]: Drupal\Core\Entity\EntityViewBuilder->build(Array)
   drupal_nginx | #7 /var/www/html/core/lib/Drupal/Core/Render/Renderer.php(378): cal" while reading response header from upstream, client: 1.2.3.4, server: server, request: "GET /mm/7/contents/add HTTP/1.1", upstream: "fastcgi://172.18.0.3:9000", host: "server:8000", referrer: "http://server:8000/"
   drupal_fpm | [22-Mar-2018 13:51:32] WARNING: [pool www] child 7 said into stderr: "#1 /var/www/html/core/lib/Drupal/Core/Extension/ModuleHandler.php(402): call_user_func_array('rdf_entity_prep...', Array)"
   drupal_nginx | 128.39.177.155 - - [22/Mar/2018:13:51:32 +0000] "GET /mm/7/contents/add HTTP/1.1" 500 85 "http://server:8000/" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:59.0) Gecko/20100101 Firefox/59.0" "-"
   drupal_fpm | 172.18.0.4 - 22/Mar/2018:13:51:31 +0000 "GET /index.php" 500
   drupal_fpm | [22-Mar-2018 13:51:32] WARNING: [pool www] child 7 said into stderr: "#2 /var/www/html/core/lib/Drupal/Core/Entity/EntityViewBuilder.php(317): Drupal\Core\Extension\ModuleHandler->invokeAll('entity_prepare_...', Array)"
   drupal_fpm | [22-Mar-2018 13:51:32] WARNING: [pool www] child 7 said into stderr: "#3 /var/www/html/core/modules/node/src/NodeViewBuilder.php(22): Drupal\Core\Entity\EntityViewBuilder->buildComponents(Array, Array, Array, 'full')"
   drupal_fpm | [22-Mar-2018 13:51:32] WARNING: [pool www] child 7 said into stderr: "#4 /var/www/html/core/lib/Drupal/Core/Entity/EntityViewBuilder.php(263): Drupal\node\NodeViewBuilder->buildComponents(Array, Array, Array, 'full')"
   drupal_fpm | [22-Mar-2018 13:51:32] WARNING: [pool www] child 7 said into stderr: "#5 /var/www/html/core/lib/Drupal/Core/Entity/EntityViewBuilder.php(220): Drupal\Core\Entity\EntityViewBuilder->buildMultiple(Array)"
   drupal_fpm | [22-Mar-2018 13:51:32] WARNING: [pool www] child 7 said into stderr: "#6 [internal function]: Drupal\Core\Entity\EntityViewBuilder->build(Array)"
   drupal_fpm | [22-Mar-2018 13:51:32] WARNING: [pool www] child 7 said into stderr: "#7 /var/www/html/core/lib/Drupal/Core/Render/Renderer.php(3..."

7. Uninstall the RDF module
8. Accessing /mm/7/contents/add does not longer crash the site. However, the existing articles seems to be gone, and navigating to /contents/add results in a "Not found" error (see attached image)