Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'entityreference_dragdrop_element_validate' not found or invalid function name

Every time I save a node with an entity reference drag & drop field, the data is saved, but I get this warning:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'entityreference_dragdrop_element_validate' not found or invalid function name in Drupal\Core\Form\FormValidator->doValidateForm() (line 281 of core/lib/Drupal/Core/Form/FormValidator.php).
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object) (Line: 239)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object) (Line: 239)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object) (Line: 239)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object, 'node_team_selection_edit_form') (Line: 119)
Drupal\Core\Form\FormValidator->validateForm('node_team_selection_edit_form', Array, Object) (Line: 571)
Drupal\Core\Form\FormBuilder->processForm('node_team_selection_edit_form', Array, Object) (Line: 314)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 74)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 576)
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}()
call_user_func_array(Object, Array) (Line: 153)
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: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 657)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)