I noticed that whenever an embedded object in a XML Block field has been deleted, a critical 500 error is raised and the page cannot be displayed.
The log show the following error :

request.CRITICAL: Uncaught PHP Exception Twig_Error_Runtime: "An exception has been thrown during the rendering of a template ("Could not find 'Content' with identifier 'array ( 'id' => 'XXXX', 'languages' => NULL, 'versionNo' => NULL, )'") in "XXXBundle:fields:content_fields.html.twig" at line 25." at /xxx/vendor/twig/twig/lib/Twig/Template.php line 148

{"exception":"[object] (Twig_Error_Runtime: A n exception has been thrown during the rendering of a template (\"Could not find 'Content' with identifier 'array (\n 'id' => 'XXXX',\n 'languages' => NULL,\n 'versionNo' => NULL,\n)'\") in \"XXXX:fields:content_fields.html.twig\" at line 25. at /XXXX/vendor/twig/twig/lib/Twig/Template.php:148, eZ\\Publish\\Core\\Base\\Exceptions\\NotFoundException: Could not find 'Content' with identifier 'array (\n 'id' => 'XXXX',\n 'languages' => NULL,\n 'versionNo' => NULL,\n)' at /home/site/www/bouygues/vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/Repository/ContentService.php:414, eZ\\Publish\\Core\\Base\\Exceptions\\NotFoundException: Could not find 'content' with identifier 'id: 8341' at /home/site/www/bouygues/vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/Persistence/Legacy/Content/Gateway/DoctrineDatabase.php:884)"}

[]

The XML Text Block field is rendered via simple ez_render_field function in the template.