That “error”, if you want to call it so, it’s fixed in 20.1. It’s in fact a changed behavior of Chrome, no longer allowing synchronous AJAX requests from a closing page.
The error has nothing to do with the signature appearing when composing a mail.
Anyway, as there is probably no more 19.1 maintenance release with 20.1 released nearly 2 month ago, you should upgrade to 20.1.
Ralf