Hello, the notice is because the variable has not been initialized. It is displayed because your server is configured to display notices and warnings (which I recommend to set off in production mode).
I will try to release an update soon to initialize variables as I’m a afraid this is not the only one which will throw notices.
Best regards,
José SAYAGO.
P.S: This notice won’t cause any issues on your site, but I know it is annoying and should be addressed.