JS Error on updating post

Hi there,
I get the following JS error every time I update a post:

seopress-block-editor.js?ver=3.7.8:36 Uncaught TypeError: Cannot read property 'length' of undefined
    at Object.success (seopress-block-editor.js?ver=3.7.8:36)
    at i (jquery.js?ver=1.12.4-wp:2)
    at Object.fireWith [as resolveWith] (jquery.js?ver=1.12.4-wp:2)
    at x (jquery.js?ver=1.12.4-wp:4)
    at XMLHttpRequest.c (jquery.js?ver=1.12.4-wp:4) 

Any ideas what is happening?