Hi Phil,
Judging by the screenshot that you shared with us, it seems that there is a Warning message and not an Error message. The warning messages will not disrupt the execution of the code.
You can simply disable the WordPress debug mode to avoid seeing the warning messages. Here is how to do so.
If you want to get into the root of the warning message I suggest that you follow the steps below:
- Ensure everything is up to date according to our version compatibility list here. Please follow the best practices when updating your theme and plugins. Click here for more information.
- Test for a plugin conflict. You can do this by deactivating all third-party plugins, and see if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Thank you.