Issue Widget does not allow more than 1 selected newsletter

If I select more than I newsletter in the issue widget, I am getting an AJAX error. Then if I save the form, I am getting the following error:

InvalidArgumentException: Value is not a valid entity. in Drupal\Core\Entity\Plugin\DataType\EntityReference->setValue() (line 106 of core/lib/Drupal/Core/Entity/Plugin/DataType/EntityReference.php).
Drupal\Core\Field\FieldItemBase->writePropertyValue('entity', Array) (Line: 234)
Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem->onChange('target_id', ) (Line: 180)
Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem->setValue(Array, ) (Line: 138)
Drupal\simplenews\Plugin\Field\FieldType\IssueItem->setValue(Array, ) (Line: 69)
Drupal\Core\TypedData\Plugin\DataType\ItemList->setValue(Array, 1) (Line: 107)
Drupal\Core\Field\FieldItemList->setValue(Array) (Line: 393)
Drupal\Core\Field\WidgetBase->extractFormValues(Object, Array, Object) (Line: 232)
Drupal\Core\Entity\Entity\EntityFormDisplay->extractFormValues(Object, Array, Object) (Line: 338)
Drupal\Core\Entity\ContentEntityForm->copyFormValuesToEntity(Object, Array, Object) (Line: 336)
Drupal\Core\Entity\EntityForm->buildEntity(Array, Object) (Line: 159)
Drupal\Core\Entity\ContentEntityForm->buildEntity(Array, Object) (Line: 190)
Drupal\Core\Entity\ContentEntityForm->validateForm(Array, Object)
call_user_func_array(Array, Array) (Line: 82)
Drupal\Core\Form\FormValidator->executeValidateHandlers(Array, Object) (Line: 273)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object, 'node_simplenews_issue_form') (Line: 118)
Drupal\Core\Form\FormValidator->validateForm('node_simplenews_issue_form', Array, Object) (Line: 578)
Drupal\Core\Form\FormBuilder->processForm('node_simplenews_issue_form', Array, Object) (Line: 321)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 91)
Drupal\Core\Controller\FormController->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: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
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: 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: 708)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)
require('/Users/peter/dev/maintainership/nrwgov/nrwgov-8.3.x-dev-2/docroot/index.php') (Line: 65)

For me it is totally unclear, what value the issue widget (\Drupal\simplenews\Plugin\Field\FieldWidget\IssueWidget) is introducing in comparison to the OptionsSelectWidget. Therefor I have patched-out the plugin class body for getting rid of the above mentioned error.