Get error message using object method not array method

[jordanleslie]

Description

This PR fixes a bug causing a compile error when the integration is disabled in Klaviyo and we try to navigate to either the consent at checkout or newsletter tabs on the klaviyo module on the M2 admin page

Jira Ticket

Manual Testing Steps

1. Switch into klaviyo account LzPPzc and disable the magento 2 integration
2. Navigate to the klaviyo newsletter and consent at checkout pages on the M2 admin page
3. You should be able to see the plugin configuration fields and not an error page with a stack trace

Pre-Submission Checklist:

• You've updated the CHANGELOG following the steps here
• Internal Only - If this is a release, please confirm the following:
  • The links in the changelog have been updated to point towards the new versions
  • The version has been incremented in the following places: module.xml and composer.json

NOTE: Please use the Changelogger cli tool to manage versioned file upgrades.

[jordanleslie]

I suspect we'll see this when the call to the list api endpoint fails but we've been unable to reproduce the issue