TypeError: implode(): Argument #1 ($pieces) must be of type array, string given in implode() (line 92 of core/modules/update/src/UpdateSettingsForm.php).

Problem/Motivation

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TypeError: implode(): Argument #1 ($pieces) must be of type array, string given in implode() (line 92 of core/modules/update/src/UpdateSettingsForm.php).
implode('', NULL) (Line: 92)
Drupal\update\UpdateSettingsForm->buildForm(Array, Object)
call_user_func_array(Array, Array) (Line: 534)
Drupal\Core\Form\FormBuilder->retrieveForm('update_settings', Object) (Line: 281)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 580)
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: 169)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 81)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 68)
Drupal\simple_oauth\HttpMiddleware\BasicAuthSwap->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
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: 49)
Asm89\Stack\Cors->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 718)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

1. drush -y cex
2. Edit update.settings.yml and change notifcations.email to a string. It is an array after a fresh install of Standard
3. drush -y cim
4. Go to admin/reports/updates/settings

Proposed resolution

Change '#default_value' => implode("\n", $notification_emails), for '#default_value' => isset($notification_emails) ? implode("\n", $notification_emails) : NULL,

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet