The Underline plugin apparently is not working, at least for me. I tried uninstalling and reinstalling the plugin, but none of that has worked. I’m working on Version 2.5b (1128) and macOS High Sierra 10.13.2.
Another strange behavior: installs the plugin twice 
I would like to know if it is possible to add to the list of plugins «visualize Strikethrough» or an option in the underline plugin.
Many thanks for keeping the work! 
Yes, there is a bug. I’ll see if I can fix it.
UPDATE: Found the problem, sent a pull request to @GeorgSeifert.
Cannot reproduce. Perhaps there are actually two installed? Take a dive into your Plugins folder, and perhaps sort it out manually.
Accepted the pull request.
Maybe you have installed it twice. Check the plugins folder.
Yes, you’re right, I have an outdated version with a different installation structure.
Current version
Outdated version
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Thanks for the pull request!