Issue #3320323 by DieterHolvoet: TypeError: strpos(): Argument #1 ($haystack)...

Issue #3320323 by DieterHolvoet: TypeError: strpos(): Argument #1 ($haystack) must be of type string, null given in strpos()

modules/mailchimp_transactional_template/src/Form/TemplateMapForm.php

@@ -133,7 +133,7 @@ class TemplateMapForm extends EntityForm {
         $form_template_name = $template_map->template_name;
       }
 
-      if ($form_template_name) {
+      if ($form_template_name && isset($template_names[$form_template_name])) {
         $regions = ['' => $this->t('-- Select --')] + $this->parseTemplateRegions($template_names[$form_template_name]->publish_code);
         $form['map_settings']['content_area'] = [
           '#type' => 'select',