If one of your layouts doesn't have regions and default regions, then adding blocks will fail with Coding error.
 
E.g. example of theme config.

  $THEME->layouts = [

      'mydashboard' => array(

      'file' => 'columns2.php',

      'regions' => array(),

      'options' => array('langmenu' => true),

    ];

 
 
And then error: 
 
_{Coding error detected, it must be fixed by a programmer: Trying to reference an unknown block region
Debug info: 
Error code: codingerror
Stack trace:} * _{line 1047 of /lib/blocklib.php: coding_exception thrown}

• _{line 821 of /lib/blocklib.php: call to block_manager->check_region_is_known()}
• _{line 892 of /lib/blocklib.php: call to block_manager->add_block()}
• _{line 1464 of /lib/blocklib.php: call to block_manager->add_block_at_end_of_default_region()}
• _{line 1390 of /lib/blocklib.php: call to block_manager->process_url_add()}
• _{line 1545 of /lib/pagelib.php: call to block_manager->process_url_actions()}
• _{line 975 of /lib/pagelib.php: call to moodle_page->starting_output()}
• _{line 1087 of /lib/outputrenderers.php: call to moodle_page->set_state()}
• _{line 164 of /my/index.php: call to core_renderer->header()}

The issue is in add_block_at_end_of_default_region function. We try to get the last element of the not existing element of array, because $defaulregion is null.