I got a bug to report... crash, closes right out of Sigil, no warning.
Don't know if this is a Sigil plugin bug, or a bug with your specific plugin.
Steps to Reproduce
1. Highlight a file in Book Browser.
Plugins > Edit > TagMechanic
2. Without doing anything, close out using the "Plugin Runner" X:
Sigil just crashes.
I tested it about 6 times:
5/6 Sigil closed with zero warning.
1/6 Sigil popped up some sort of XML parsing warning (but I forgot to screenshot it). Sigil then crashed soon after.