DWC Plugins cannot be installed

[MintyTrebor]

Trying to install a DWC plugin I get an error:

Failed to decompress files
Cannot read property 'find' of null

Log:
========== Started ==========
Tornado:.1845398 - ##.##.##.## - GET - /rr_connect?password=reprap&time=2021-5-17T13%3A18%3A9
Tornado:.2179363 - ##.##.##.## - GET - /rr_filelist?dir=0%3A%2Fmacros&first=0
Tornado:9.229812 - ##.##.##.## - GET - /rr_filelist?dir=0%3A%2Fsys&first=0
Tornado:.4704661 - ##.##.##.## - GET - /BtnCmd
Tornado:.4810545 - ##.##.##.## - GET - /rr_disconnect
Tornado:.5994647 - ##.##.##.## - GET - /js/app.56671d18.js
Tornado:8.921883 - ##.##.##.## - GET - /rr_connect?password=reprap&time=2021-5-17T13%3A18%3A28
Tornado:.9850492 - ##.##.##.## - GET - /rr_download?name=0%3A%2Fsys%2Fdwc-plugins.json
Tornado:.0389519 - ##.##.##.## - GET - /rr_download?name=0%3A%2Fsys%2Fdwc-settings.json
Tornado:.0448318 - ##.##.##.## - GET - /img/icons/favicon-32x32.png
Tornado:9.064854 - ##.##.##.## - GET - /css/HeightMap.389e8521.css
Tornado:.0786698 - ##.##.##.## - GET - /js/HeightMap.7361cf24.js
Tornado:.1083689 - ##.##.##.## - GET - /rr_filelist?dir=0%3A%2Fsys&first=0

I understand this may be a niche use case, and you may not wish to support it.

plugin here : https://github.com/MintyTrebor/BtnCmd/releases/download/Alpha_0.8.18/BtnCmd_DWC3.2_Alpha.0.8.18.zip

For background I am the author of the plugin, and I am in the process of building a new printer which will be using Klipper. I was excited to see I could use DWC as it meant I can continue to use my plugin once I have completed the build. The plugin does work if I install a custom build of the whole of DWC site with the plugin already embedded, so I know that the plugin will function with your solution once installed.

EDIT:
Forgot to add that I also did a test with DWC 3.3rc2, and had similar results. DWC 3.3rc2 operated normally with your socket, but the plugin installation failed (as before - a build with the plugin already included ran fine).