Nav Walker current menu item not displaying

Since it is working as you expect in the header.php file (but not in sidebar.php),
you could add this into header.php:

global $my_nav;
$my_nav= wp_nav_menu( array(
          'menu'            => 'main-menu',
          'container' => '',
          'container_class' => 'navbar-blue',
          'menu_class'      => 'nav',
          'menu_id' => 'main-menu',
          'walker' => new Bootstrapwp_Walker_Nav_Menu(),
          'echo' => 0,
      ) ); ?>

and then in your sidebar.php file you could add

global $my_nav;
echo $my_nav;

to display the menu.