Throw exception in \Drupal\Core\StreamWrapper\PrivateStream::getDirectoryPath() when basepath() is null

In \Drupal\Core\CoreServiceProvider::register() we do...

    // Only register the private file stream wrapper if a file path has been set.
    if (Settings::get('file_private_path')) {
      $container->register('stream_wrapper.private', 'Drupal\Core\StreamWrapper\PrivateStream')
        ->addTag('stream_wrapper', ['scheme' => 'private']);
    }

Therefore the code that relies on \Drupal\Core\StreamWrapper\PrivateStream::getDirectoryPath() reasonably expected this to return a string. If the private stream wrapper is incorrectly registered prior to the file_private_path being set we should throw a logic exception since it is just not supposed to work this way.

Also we could consider changing
Settings::get('file_private_path') to \Drupal\Core\StreamWrapper\PrivateStream::basePath()
Settings::get('file_public_path') to \Drupal\Core\StreamWrapper\PublicStream::basePath()
We could save some duplicated code in system_requirements() and have less hard-coded strings lying about.

@andypost well that's commented out - so by default it is not set :) i.e NULL. And it is not set by the installer.