With no blocks, clicking on the Custom Blocks tab crashes site

I'm seeing:

The website encountered an unexpected error. Please try again later.

When reviewing the log, I see:

Drupal\Component\Plugin\Exception\PluginNotFoundException: The "block_content" entity type does not exist. in Drupal\Core\Entity\EntityTypeManager->getDefinition() (line 150 of /var/aegir/platforms/example_platform/web/core/lib/Drupal/Core/Entity/EntityTypeManager.php).

It would certainly be nicer to land on the tab with a message like, "You don't have any custom blocks." rather than trip the exception handler.

Aside: It would be nice if subsystems didn't need to be installed as dependencies if they're not being used. For example, I only care about Taxonomy, so I shouldn't have to enable `menu_link_content` and `block`.