Thread pool is gone

if i click on tf-viewer link in the top left corner i get taken to the root URL, and am shown :

`Thread pool is gone`

docker logs show
`tf-viewer | thread 'actix-web' panicked at 'called `Option::unwrap()` on a `None` value', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/staticmap-0.3.2/src/tools/line.rs:105:42`

Reproduce the failure by clicking the tf-viewer link in the top-left corner and inspect the Docker logs. Start with the panic reported in staticmap-0.3.2/src/tools/line.rs:105:42, then trace how the root URL reaches that code. Done means the link no longer shows “Thread pool is gone” and the actix-web thread does not panic.

Written by the indexing model from the issue text.