Entity reference map configuration page returns errors if not being set.
Navigate to /admin/structure/types/manage/{entity_type}/map page of the content type without set entity reference map configurations.
Warning: Undefined array key "article" in Drupal\entity_reference_map\Form\MapForm->getCheckboxDefaultValue() (line 354 of modules/contrib/dmytro-aragorn-3259790/src/Form/MapForm.php).
Drupal\entity_reference_map\Form\MapForm->getCheckboxDefaultValue('article', 'forward_reference', 'node.article.field_expert_reference') (Line: 159)
Drupal\entity_reference_map\Form\MapForm->buildForwardReferenceCheckboxes(Array) (Line: 134)
Drupal\entity_reference_map\Form\MapForm->buildForm(Array, Object, Object)
call_user_func_array(Array, Array) (Line: 536)
Drupal\Core\Form\FormBuilder->retrieveForm('entity_reference_map.map_form', Object) (Line: 283)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 580)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 121)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 163)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 74)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
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: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 1, 1) (Line: 686)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)
Warning: Trying to access array offset on value of type null in Drupal\entity_reference_map\Form\MapForm->getCheckboxDefaultValue() (line 354 of modules/contrib/dmytro-aragorn-3259790/src/Form/MapForm.php).
Drupal\entity_reference_map\Form\MapForm->getCheckboxDefaultValue('article', 'forward_reference', 'node.article.field_expert_reference') (Line: 159)
Drupal\entity_reference_map\Form\MapForm->buildForwardReferenceCheckboxes(Array) (Line: 134)
Drupal\entity_reference_map\Form\MapForm->buildForm(Array, Object, Object)
call_user_func_array(Array, Array) (Line: 536)
Drupal\Core\Form\FormBuilder->retrieveForm('entity_reference_map.map_form', Object) (Line: 283)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 580)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 121)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 163)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 74)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
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: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 1, 1) (Line: 686)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)
Add a proper check in getCheckboxDefaultValue method in MapForm class.