Fehler bei D9 Upgrade

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TypeError: Illegal offset type in Drupal\migrate_drupal\MigrationState->buildDiscoveredDestinationsBySource() (line 349 of core/modules/migrate_drupal/src/MigrationState.php).

Drupal\migrate_drupal\MigrationState->buildDiscoveredDestinationsBySource() (Line: 301)
Drupal\migrate_drupal\MigrationState->buildUpgradeState() (Line: 244)
Drupal\migrate_drupal\MigrationState->getUpgradeStates() (Line: 133)
Drupal\migrate_drupal_ui\Form\ReviewForm->buildForm()
call_user_func_array() (Line: 531)
Drupal\Core\Form\FormBuilder->retrieveForm() (Line: 278)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 73)
Drupal\Core\Controller\FormController->getContentResult()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 564)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 158)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 58)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 23)
Stack\StackedHttpKernel->handle() (Line: 709)
Drupal\Core\DrupalKernel->handle() (Line: 19