Fixed the issue of [Login Destination] Route Error with get route needed the link to be Routed

Thank you for reporting!
I think our best option is to patch the module after
944d78fbf01853244d8e01f63b41dc37d5f99e50

http://cgit.drupalcode.org/login_destination/log/?h=8.x-1.x

We could add the patch in #2931356: Fixed Route Error as links should be routed ( is routed ) before we get their route name as you reported that in the private.

After Reading Omar Alahmed's comment on this link
https://api.drupal.org/comment/63563#comment-63563
links should be Routed.

Applying the Patch #5 2931356-5.patch in #2931356: Fixed Route Error as links should be routed ( is routed ) before we get their route name for Varbase 8.4.13

And you could add the patch in your varbase-project patches as follows:

   "patches": {
      "drupal/login_destination": {
        "Issue #2931356: Fixes Route Error.":
        "https://www.drupal.org/files/issues/2931356-5.patch"
      }
   }

FYI, I received that error in the combination of Login Destination 8.x-1.x-dev on Drupal core 8.5-dev:

UnexpectedValueException: External URLs do not have an internal route name. in Drupal\Core\Url->getRouteName() (line 555 of /home/sitename/web/core/lib/Drupal/Core/Url.php).

And in that case, thhe patch of #2931356: Fixed Route Error as links should be routed ( is routed ) before we get their route name worked for me.

#2934280: Updated [Login Destination] module from 8.1.x-dev-944d78f to 8.1.x-dev-54be8b8