Cannot add a file field to a content type

In Drupal 8.5.1 (starting all over from scratch), I need to make a content type with a file upload. As soon as I try to configure this field, I get a “server encountered an unknown error” message, and a long log message. What am I doing wrong?

Location http://localhost/stelnews/admin/structure/types/manage/issue/fields/node.issue.field_pdffile?destinations%5B0%5D%5Broute_name%5D=entity.node.field_ui_fields&destinations%5B0%5D%5Broute_parameters%5D%5Bfield_config%5D=node.issue.field_pdffile&destinations%5B0%5D%5Broute_parameters%5D%5Bnode_type%5D=issue&destinations%5B1%5D=%2Fstelnews%2Fadmin%2Fstructure%2Ftypes%2Fmanage%2Fissue%2Ffields%2Fadd-field
Referrer http://localhost/stelnews/admin/structure/types/manage/issue/fields/node.issue.field_pdffile?destinations%5B0%5D%5Broute_name%5D=entity.node.field_ui_fields&destinations%5B0%5D%5Broute_parameters%5D%5Bfield_config%5D=node.issue.field_pdffile&destinations%5B0%5D%5Broute_parameters%5D%5Bnode_type%5D=issue&destinations%5B1%5D=/stelnews/admin/structure/types/manage/issue/fields/add-field
Message Error: Call to a member function uuid() on null in DrupalCoreFieldEntityReferenceFieldItemList->defaultValuesFormSubmit() (line 126 of /srv/www/htdocs/stelnews/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php) #0 /srv/www/htdocs/stelnews/core/modules/field_ui/src/Form/FieldConfigEditForm.php(169): DrupalCoreFieldEntityReferenceFieldItemList->defaultValuesFormSubmit(Array, Array, Object(DrupalCoreFormFormState)) #1 [internal function]: Drupalfield_uiFormFieldConfigEditForm->submitForm(Array, Object(DrupalCoreFormFormState)) #2 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/Form/FormSubmitter.php(111): call_user_func_array(Array, Array) #3 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/Form/FormSubmitter.php(51): DrupalCoreFormFormSubmitter->executeSubmitHandlers(Array, Object(DrupalCoreFormFormState)) #4 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/Form/FormBuilder.php(585): DrupalCoreFormFormSubmitter->doSubmitForm(Array, Object(DrupalCoreFormFormState)) #5 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/Form/FormBuilder.php(314): DrupalCoreFormFormBuilder->processForm(‘field_config_ed…’, Array, Object(DrupalCoreFormFormState)) #6 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/Controller/FormController.php(74): DrupalCoreFormFormBuilder->buildForm(‘field_config_ed…’, Object(DrupalCoreFormFormState)) #7 [internal function]: DrupalCoreControllerFormController->getContentResult(Object(SymfonyComponentHttpFoundationRequest), Object(DrupalCoreRoutingRouteMatch)) #8 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array(Array, Array) #9 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/Render/Renderer.php(582): DrupalCoreEventSubscriberEarlyRenderingControllerWrapperSubscriber->DrupalCoreEventSubscriber{closure}() #10 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(124): DrupalCoreRenderRenderer->executeInRenderContext(Object(DrupalCoreRenderRenderContext), Object(Closure)) #11 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): DrupalCoreEventSubscriberEarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) #12 /srv/www/htdocs/stelnews/vendor/symfony/http-kernel/HttpKernel.php(151): DrupalCoreEventSubscriberEarlyRenderingControllerWrapperSubscriber->DrupalCoreEventSubscriber{closure}() #13 /srv/www/htdocs/stelnews/vendor/symfony/http-kernel/HttpKernel.php(68): SymfonyComponentHttpKernelHttpKernel->handleRaw(Object(SymfonyComponentHttpFoundationRequest), 1) #14 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/StackMiddleware/Session.php(57): SymfonyComponentHttpKernelHttpKernel->handle(Object(SymfonyComponentHttpFoundationRequest), 1, true) #15 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): DrupalCoreStackMiddlewareSession->handle(Object(SymfonyComponentHttpFoundationRequest), 1, true) #16 /srv/www/htdocs/stelnews/core/modules/page_cache/src/StackMiddleware/PageCache.php(99): DrupalCoreStackMiddlewareKernelPreHandle->handle(Object(SymfonyComponentHttpFoundationRequest), 1, true) #17 /srv/www/htdocs/stelnews/core/modules/page_cache/src/StackMiddleware/PageCache.php(78): Drupalpage_cacheStackMiddlewarePageCache->pass(Object(SymfonyComponentHttpFoundationRequest), 1, true) #18 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupalpage_cacheStackMiddlewarePageCache->handle(Object(SymfonyComponentHttpFoundationRequest), 1, true) #19 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(50): DrupalCoreStackMiddlewareReverseProxyMiddleware->handle(Object(SymfonyComponentHttpFoundationRequest), 1, true) #20 /srv/www/htdocs/stelnews/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): DrupalCoreStackMiddlewareNegotiationMiddleware->handle(Object(SymfonyComponentHttpFoundationRequest), 1, true) #21 /srv/www/htdocs/stelnews/core/lib/Drupal/Core/DrupalKernel.php(664): StackStackedHttpKernel->handle(Object(SymfonyComponentHttpFoundationRequest), 1, true) #22 /srv/www/htdocs/stelnews/index.php(19): DrupalCoreDrupalKernel->handle(Object(SymfonyComponentHttpFoundationRequest)) #23 {main}.
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Source: https://www.drupal.org/taxonomy/term/22/feed