Hello Paddy,
First, let me start by mentioning that this is not an Error, but a PHP Warning, instead. The difference is that the warning are not preventative for the execution of the php code, they are rather a notification of an incompatibility or a notice how some part of the code could be improved. This does not mean that the code is incorrect though or that it needs fixing. In many cases, such a code is implemented on purpose – to make it more widely compatible even with sites that run on older PHP Versions.
I tried to reproduce the reported waring both by updating a theme on a test site and also by deactivating and re-activating the Divi theme on your oldstyle site and then triggering checks in both plugins – SiteGround Optimizer and Security. None of these actions generated the Warning – it seems to occur at some very corner case scenario and you should safely disregard it.
Regards,
Plamen
