Issue #3378860: Upgrade leads to array_merge(): Argument #2 must be of type array, null given

src/Processor/MailerConfigOverride.php

@@ -7,6 +7,8 @@ use Drupal\Core\Config\ConfigFactoryOverrideInterface;
 use Drupal\Core\Config\StorageInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 
+use function PHPUnit\Framework\isNull;
+
 /**
  * Symfony Mailer configuration override.
  */
@@ -90,6 +92,10 @@ class MailerConfigOverride implements ConfigFactoryOverrideInterface {
       $builderManager = \Drupal::service('plugin.manager.email_builder');
 
       foreach ($builderManager->getDefinitions() as $definition) {
+        if (isNull($definition['config_overrides'])) {
+          $definition = [];
+          return;
+        }
         $this->configOverrides = array_merge($this->configOverrides, $definition['config_overrides']);
       }
     }