Fatal error on save when "Authored by" or "Fieldset" fields are deactivated in form mode

Problem/Motivation

Steps to reproduce:

• Install module
• Create new site settings entity type
• Move the Authored by field to the deactivated section in the form display configuration
• Create new instance of the entity with the new entity type, save --> error

Error:

Auf der Website ist ein unvorhergesehener Fehler aufgetreten. Bitte versuchen Sie es später nochmal.</br></br><em class="placeholder">Drupal\Core\Entity\EntityStorageException</em>: SQLSTATE[23000]: Integrity constraint violation: 1048 Column &#039;user_id&#039; cannot be null: INSERT INTO {site_setting_entity_field_data} (id, type, langcode, user_id, name, fieldset, status, created, changed, default_langcode) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9); Array
(
    [:db_insert_placeholder_0] =&gt; 4
    [:db_insert_placeholder_1] =&gt; footer_bottom_text
    [:db_insert_placeholder_2] =&gt; de
    [:db_insert_placeholder_3] =&gt; 
    [:db_insert_placeholder_4] =&gt; Footer bottom text
    [:db_insert_placeholder_5] =&gt; Footer
    [:db_insert_placeholder_6] =&gt; 1
    [:db_insert_placeholder_7] =&gt; 1562843674
    [:db_insert_placeholder_8] =&gt; 1562843674
    [:db_insert_placeholder_9] =&gt; 1
)
 in <em class="placeholder">Drupal\Core\Entity\Sql\SqlContentEntityStorage-&gt;save()</em> (line <em class="placeholder">847</em> of <em class="placeholder">core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php</em>). <pre class="backtrace">Drupal\Core\Database\Statement-&gt;execute(Array, Array) (Line: 631)
Drupal\Core\Database\Connection-&gt;query(&#039;INSERT INTO {site_setting_entity_field_data} (id, type, langcode, user_id, name, fieldset, status, created, changed, default_langcode) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9)&#039;, Array, Array) (Line: 358)
Drupal\Core\Database\Driver\mysql\Connection-&gt;query(&#039;INSERT INTO {site_setting_entity_field_data} (id, type, langcode, user_id, name, fieldset, status, created, changed, default_langcode) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9)&#039;, Array, Array) (Line: 32)
Drupal\Core\Database\Driver\mysql\Insert-&gt;execute() (Line: 1052)
Drupal\Core\Entity\Sql\SqlContentEntityStorage-&gt;saveToSharedTables(Object) (Line: 990)
Drupal\Core\Entity\Sql\SqlContentEntityStorage-&gt;doSaveFieldItems(Object) (Line: 657)
Drupal\Core\Entity\ContentEntityStorageBase-&gt;doSave(NULL, Object) (Line: 449)
Drupal\Core\Entity\EntityStorageBase-&gt;save(Object) (Line: 838)
Drupal\Core\Entity\Sql\SqlContentEntityStorage-&gt;save(Object) (Line: 394)
Drupal\Core\Entity\EntityBase-&gt;save() (Line: 79)
Drupal\site_settings\Form\SiteSettingEntityForm-&gt;save(Array, Object)
call_user_func_array(Array, Array) (Line: 111)
Drupal\Core\Form\FormSubmitter-&gt;executeSubmitHandlers(Array, Object) (Line: 51)
Drupal\Core\Form\FormSubmitter-&gt;doSubmitForm(Array, Object) (Line: 590)
Drupal\Core\Form\FormBuilder-&gt;processForm(&#039;site_setting_entity_footer_bottom_text_form&#039;, Array, Object) (Line: 319)
Drupal\Core\Form\FormBuilder-&gt;buildForm(&#039;site_setting_entity_footer_bottom_text_form&#039;, Object) (Line: 61)
Drupal\Core\Entity\EntityFormBuilder-&gt;getForm(Object) (Line: 99)
Drupal\site_settings\Controller\SiteSettingEntityAddController-&gt;addForm(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber-&gt;Drupal\Core\EventSubscriber\{closure}() (Line: 582)
Drupal\Core\Render\Renderer-&gt;executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber-&gt;wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber-&gt;Drupal\Core\EventSubscriber\{closure}() (Line: 151)
Symfony\Component\HttpKernel\HttpKernel-&gt;handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel-&gt;handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session-&gt;handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle-&gt;handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache-&gt;pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache-&gt;handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware-&gt;handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware-&gt;handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel-&gt;handle(Object, 1, 1) (Line: 693)
Drupal\Core\DrupalKernel-&gt;handle(Object) (Line: 19)
</pre>

Some screenshots:

Proposed resolution

Support moving authored by field to deactivated section. Maybe just get the current user, and use it as the author, instead of using the form value, if it's always beeing hidden anyway.

Remaining tasks

Fix bug

User interface changes

None.

API changes

tbd.

Data model changes

None.