Not sure why anyone would do this out of testing for issues, but automatically fixing an invalid xpgt file causes a crash.
Create a file using your favourite editor called junk.xpgt and put the single word "hello" in it.
Right click on Styles and select add existing files and select junk.xpgt
Double click on junk.xpgt to open it in a tab
Click on the other tab
->You're prompted to fix automatically or manually
Select fix automatically
->Sigil crashes with the message:
Panic - tree has lost its integrity