Warning: Undefined property: stdClass::$gradient

Hello Balbooa,
On one of my sites, I use the Zero app, to allow my clients to maintain their own events and news. Today, they raised an error. Whenever they try to save a new entry, they see the pop-up:
"
Warning: Undefined property: stdClass::$gradient in [REMOVED]/public_html/components/com_gridbox/src/Helper/CssHelper.php on line 236
Page has been saved successfully
"
(I've removed the path)

Because the website was recently updated to the latest Balbooa and Joomala, versions, I turned off the CSS / HTML minify and browser cache - just in case. When I tried to save these changes, I also saw the same error.

I've cleared the cache on my host. Joomla reports all overrides are up to date.

Could you please take a look. The URL is:
https://www.rejigged.co.uk

Many thanks
Marc