FPDoc Editor is open.
Does the issue go away, if you close it (and maybe restart the IDE (and keep it closed)?
The fact that is is empty, only means there is no entry in the file for that particular keyword. There may still be a xml file open.
I don't know if FpDoc would (or should) try to (re-)save the file, if you haven't edited any of the entries (assuming you haven't). But easiest way to find out is to close it (restart the IDE with it closed before, so it really is closed) and see if the error still happens.