The next page of the parent menu displays the submenu

Added restrictions:
Get the active first-level menu under the menu and avoid buttons of other first-level menus on the start and end menus

Added restrictions:
Get the active first-level menu under the menu and avoid buttons of other first-level menus on the start and end menus
Fixed():
Undefined array key "main" in Drupal\menu_pager\Plugin\Block\MenuBlock->menuPagerGetNavigation()