update-alternatives: error: alternative path /usr/share/gnome-shell/theme/gnome-shell.css doesn't exist

[Alan Pope 🍺🐧🐱 🦄]

Just updated my system after a few days of not doing so and got:-

Setting up gnome-session (3.25.90-0ubuntu3) ...

update-alternatives: error: alternative path /usr/share/gnome-shell/theme/gnome-shell.css doesn't exist
dpkg: error processing package gnome-session (--configure):
  subprocess installed post-installation script returned error exit status 2

If I try to re-install it fails too:-

$ sudo apt install gnome-session --reinstall
[sudo] password for alan:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
  javascript-common libjs-excanvas libllvm4.0:i386 liblttng-ust-ctl2
  liblttng-ust0 libubuntu-app-launch4 liburcu6 linux-headers-4.11.0-13
  linux-headers-4.11.0-13-generic linux-image-4.11.0-13-generic
  linux-image-extra-4.11.0-13-generic linux-signed-image-4.11.0-13-generic
  mercurial mercurial-common ubuntu-app-launch
Use 'sudo apt autoremove' to remove them.
0 to upgrade, 0 to newly install, 1 reinstalled, 0 to remove and 0 not to upgrade.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
E: Internal Error, No file name for gnome-session:amd64

[Sebastien Bacher (seb128)]

that's fixed in https://bugs.launchpad.net/ubuntu/+source/gnome-session/3.25.90-0ubuntu4