New interest groups can be NULL

Alternatively it should return an empty array instead of NULL.

That actually still didn't fix the error, because it turns out it can be FALSE as well. At least that is what is on the entity, I assume it is the same after an entity is saved.

$user->get('field_mailchimp_field')->get(0);
=> Drupal\mailchimp_lists\Plugin\Field\FieldType\MailchimpListsSubscription {#8216
   value: [
     "subscribe" => "0",
     "interest_groups" => false,
   ],
 }

Notice "subscribe" is also "0" rather than FALSE, so maybe this is data from an earlier version of the module which is causing it to have errors. I don't know if there should be something done to fix the data, e.g. to make it like this:

[
  "subscribe" => false,
  "interest_groups" => [],
]

But here is a patch to just fix the PHP error.

@Rob230 I wonder if this patch might resolve the issue? Just adding a check over that entire block of code to see if the new_interest_groups exists and is an array.