Array to string conversion in webform/src/Utility/WebformOptionsHelper.php on line 59

+++ b/src/Plugin/WebformHandler/WebformMailChimpHandler.php
@@ -125,7 +125,7 @@ class WebformMailChimpHandler extends WebformHandlerBase {
       '#empty_option' => $this->t('- Select an option -'),
-      '#default_value' => $this->configuration['list'],
+      '#default_value' => ($this->configuration['list'] ?: null),
       '#options' => $options,

The problem is that the default value for list in defaultConfiguration is set to an empty array instead of NULL, change that and it should work.

Ok, I also understand now that this only happens when your mailchimp account has no list yet, which is quite unusual. And you can't use this module in that scenario, you need a list, the other option still needs to pick a valid list, just dynamically. Either way, the correct default value is NULL indeed.

Committed.

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