PHP Notice: Undefined index: #id when used with Context module

*sigh* The differing rendering of block.html.twig between Context module and core Drupal is annoying.

With menu_block 8.x-1.3, I'm seeing this PHP notice:

Notice: Undefined index: #id in menu_block_theme_suggestions_block_alter()

because $variables['elements']['#id'] doesn't exist when Context module is rendering a block.