I quick (pro) tip:
During the creation of an admin navigation you are asked for an „Admin Navigation ID“.
It sounds trivial but this ID is used in your "WhatEver_Route_PrefixAdmin_Something' as the third parameter of "$router->getRouteMatch()"
So that means if you create an admin navigation with the ID of "test123" the corresponding line in AdminPrefix/Something.php has to look like this: return $router->getRouteMatch('WhatEver_ControllerAdmin_Something', $action, 'test123');
What happens if you don't pay attention to this is that the AdminController you are linking to will always open in the home tab and the breadcrumb won't work.
It took me twenty minutes to figure out so I thought I'd share it with you.