Hello,
In order to fully understand why you have that message, we should have an access to your configuration (database).
It looks like you have a vendor with wrong "access" stored in the database.
The best is to edit the config.php file (that you have in your error message) and replace the line
$vendor_access = $vendor->vendor_access;
By
$vendor_access = $vendor->vendor_access;
if(empty($vendor_access) || !is_array($vendor_access))
$vendor_access = array();
And it should remove the warning.
Regards,