MandrillTemplateMap is missing config_export definition

Problem/Motivation

Using Mandrill on Drupal 9 successfully with patch #18 from here: https://www.drupal.org/project/mandrill/issues/3137614

However, the Mandrill Template modules does not seem to be working.
Everytime I click on "Save" when trying to add a template I get this error:

The website encountered an unexpected error. Please try again later.

Drupal\Core\Config\Schema\SchemaIncompleteException: Entity type 'Drupal\mandrill_template\Entity\MandrillTemplateMap' is missing 'config_export' definition in its annotation in Drupal\Core\Config\Entity\ConfigEntityBase->toArray() (line 251 of core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php).
Drupal\Core\Entity\Plugin\DataType\EntityAdapter->toArray() (Line: 717)
we_megamenu_entity_presave(Object, 'mandrill_template_map')
call_user_func_array('we_megamenu_entity_presave', Array) (Line: 403)
Drupal\Core\Extension\ModuleHandler->invokeAll('entity_presave', Array) (Line: 349)
Drupal\Core\Config\Entity\ConfigEntityStorage->invokeHook('presave', Object) (Line: 495)
Drupal\Core\Entity\EntityStorageBase->doPreSave(Object) (Line: 449)
Drupal\Core\Entity\EntityStorageBase->save(Object) (Line: 263)
Drupal\Core\Config\Entity\ConfigEntityStorage->save(Object) (Line: 339)
Drupal\Core\Entity\EntityBase->save() (Line: 591)
Drupal\Core\Config\Entity\ConfigEntityBase->save() (Line: 166)
Drupal\mandrill_template\Form\MandrillTemplateMapForm->save(Array, Object)
call_user_func_array(Array, Array) (Line: 113)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 51)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 593)
Drupal\Core\Form\FormBuilder->processForm('mandrill_template_map_add_form', Array, Object) (Line: 144)
Drupal\autosave_form\Form\AutosaveFormBuilder->processForm('mandrill_template_map_add_form', Array, Object) (Line: 321)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 97)
Drupal\autosave_form\Form\AutosaveFormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object) (Line: 39)
Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 573)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 158)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 57)
Drupal\advban\AdvbanMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\ban\BanMiddleware->handle(Object, 1, 1) (Line: 115)
Drupal\shield\ShieldMiddleware->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 706)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

Install drupal/mandrill using composer and add mandrill library using composer.
Enable Mandrill, test... it works great.
Now enable Mandrill Template, go to configuration and try to add a new template map.
Then you will see the error.

User interface changes

I tried creating a new template in Mandrill website, tried creating template map with email keys and with "none" email keys. All fail.

Wondering if this a Drupal 9 problem or if it also shows in Drupal 8.

Anyone having this issue?

Thank you.