Warning: foreach() argument must be of type array|object, null given in Drupal\Core\Render\Element\Checkboxes::valueCallback()

Problem/Motivation

Warning: foreach() argument must be of type array|object, null given in Drupal\Core\Render\Element\Checkboxes::valueCallback() (line 113 of core/lib/Drupal/Core/Render/Element/Checkboxes.php).
Drupal\Core\Render\Element\Checkboxes::valueCallback(Array, , Object)
call_user_func_array(Array, Array) (Line: 1287)
Drupal\Core\Form\FormBuilder->handleInputElement('advanced_pwa_subscription_form', Array, Object) (Line: 1005)
Drupal\Core\Form\FormBuilder->doBuildForm('advanced_pwa_subscription_form', Array, Object) (Line: 1075)
Drupal\Core\Form\FormBuilder->doBuildForm('advanced_pwa_subscription_form', Array, Object) (Line: 579)
Drupal\Core\Form\FormBuilder->processForm('advanced_pwa_subscription_form', Array, Object) (Line: 325)
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: 592)
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: 181)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->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: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 1, 1) (Line: 704)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

1. Install module.
2. Go to: /admin/config/system/advanced-pwa/config-subscription.
3. Catch warning.