This extension is no longer supported, and it works on Burp 1.7.x only. You can download the 1.7 version from Portswigger website.
Originally posted by @ikkisoft in #11 (comment)
I tried with 1.7.37 and this error was returned by the extension:
java.lang.NullPointerException at burp.CustomMenuItem.menuItemClicked(BurpExtender.java:153) at burp.he.run(Unknown Source) at java.lang.Thread.run(Thread.java:748)