Exception for unsaved group when it must not

Problem/Motivation

When we use field_permissions_group, we get an exception when creating a group (see backtrace below).

The root reason is GroupContentStorage::loadByGroup() which throws on an unsaved Group, but its PHPDoc contract does not announce a @throws.

Major as of WSOD.

Backtrace of field_permissions_group check
Drupal\Core\Entity\EntityStorageException: Cannot load GroupContent entities for an unsaved group. in Drupal\group\Entity\Storage\GroupContentStorage->loadByGroup() (line 71 of modules/contrib/group/src/Entity/Storage/GroupContentStorage.php).

Drupal\group\GroupMembershipLoader->load(Object, Object) (Line: 190)
Drupal\group\Entity\Group->getMember(Object) (Line: 73)
Drupal\field_permissions_group\Plugin\FieldPermissionType\CustomGroupAccess->hasFieldAccess('edit', Object, Object) (Line: 174)
Drupal\field_permissions\FieldPermissionsService->getFieldAccess('edit', Object, Object, Object) (Line: 39)
field_permissions_entity_field_access('edit', Object, Object, Object)
call_user_func_array('field_permissions_entity_field_access', Array) (Line: 392)
Drupal\Core\Extension\ModuleHandler->invoke('field_permissions', 'entity_field_access', Array) (Line: 347)
Drupal\Core\Entity\EntityAccessControlHandler->fieldAccess('edit', Object, Object, Object, ) (Line: 154)
Drupal\Core\Field\FieldItemList->access('edit') (Line: 179)
Drupal\Core\Entity\Entity\EntityFormDisplay->buildForm(Object, Array, Object) (Line: 125)
Drupal\Core\Entity\ContentEntityForm->form(Array, Object) (Line: 149)
Drupal\Core\Entity\EntityForm->buildForm(Array, Object)
call_user_func_array(Array, Array) (Line: 532)
Drupal\Core\Form\FormBuilder->retrieveForm('group_mitgliedschaft_add_form', Object) (Line: 278)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 61)
Drupal\Core\Entity\EntityFormBuilder->getForm(Object, 'add', Array) (Line: 124)
Drupal\group\Entity\Controller\GroupController->addForm(Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 573)
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}() (Line: 151)
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: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 708)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

See above.

Proposed resolution

Make GroupContentStorage::loadByGroup adhere to the contract and return an empty array.

Remaining tasks

Patch, check, commit.

User interface changes

None.

API changes

The code will do what the API says.

Data model changes

None.