Warning: Undefined array key "interest_groups" in MailchimpListsSelectWidget->setupInterestGroups()

I updated the MR to check that $instance->getValue()['interest_groups'] is an array since $groups_default will be passed to a function that expects an array.

Do you want to verify that this works on your installation?

I can confirm that the updated code works on my installation as well. Changing the status to RTBC

Automatically closed - issue fixed for 2 weeks with no activity.