Add Schema.org content type shows Deprecated function: Using null as an array offset is deprecated, use an empty string instead in Drupal\schemadotorg\SchemaDotOrgEntityFieldManager->fieldTypeExists()

Problem/Motivation

Trying to add a new Schema.org content type shows:
Deprecated function: Using null as an array offset is deprecated, use an empty string instead in Drupal\schemadotorg\SchemaDotOrgEntityFieldManager->fieldTypeExists() (line 557 of modules/contrib/schemadotorg/src/SchemaDotOrgEntityFieldManager.php).

Backtrace:

Drupal\schemadotorg\SchemaDotOrgEntityFieldManager->fieldTypeExists() (Line: 437)
Drupal\schemadotorg\SchemaDotOrgEntityFieldManager->getSchemaPropertyFieldTypes() (Line: 175)
Drupal\schemadotorg\SchemaDotOrgEntityFieldManager->getPropertyFieldTypeOptions() (Line: 755)
Drupal\schemadotorg_ui\Form\SchemaDotOrgUiMappingForm->buildSchemaPropertyFieldForm() (Line: 630)
Drupal\schemadotorg_ui\Form\SchemaDotOrgUiMappingForm->buildSchemaPropertiesForm() (Line: 437)
Drupal\schemadotorg_ui\Form\SchemaDotOrgUiMappingForm->buildFieldTypeForm() (Line: 183)
Drupal\schemadotorg_ui\Form\SchemaDotOrgUiMappingForm->form() (Line: 108)
Drupal\Core\Entity\EntityForm->buildForm()
call_user_func_array() (Line: 554)
Drupal\Core\Form\FormBuilder->retrieveForm() (Line: 303)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 73)
Drupal\Core\Controller\FormController->getContentResult()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->{closure:Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber::wrapControllerExecutionInRenderContext():121}() (Line: 634)
Drupal\Core\Render\Renderer::{closure:Drupal\Core\Render\Renderer::executeInRenderContext():634}()
Fiber->start() (Line: 635)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 121)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->{closure:Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber::onController():96}() (Line: 183)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 53)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle() (Line: 118)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 92)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 53)
Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 54)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 745)
Drupal\Core\DrupalKernel->handle() (Line: 19)

Steps to reproduce

1. Go to /admin/config/schemadotorg/mappings
2. Click the +Add Schema.org mapping button
3. Click Create new Schema.org mapping > Content type
4. Click any type, e.g. WebPage

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes