On fresh d8 install of latest code, navigating to /block/add leads to this error:
Fatal error: Call to a member function get() on a non-object in /Library/WebServer/Documents/drupal/core/modules/block/custom_block/lib/Drupal/custom_block/Controller/CustomBlockController.php on line 105
There are some notices as well:
Notice: Undefined variable: request in Drupal\custom_block\Controller\CustomBlockController->addForm() (line 105 of core/modules/block/custom_block/lib/Drupal/custom_block/Controller/CustomBlockController.php).
Notice: Trying to get property of non-object in Drupal\custom_block\Controller\CustomBlockController->addForm() (line 105 of core/modules/block/custom_block/lib/Drupal/custom_block/Controller/CustomBlockController.php).