Warning: foreach() argument must be of type array|object, null given

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: 1285)
Drupal\Core\Form\FormBuilder->handleInputElement('nginx_proxy_settings_form', Array, Object) (Line: 1003)
Drupal\Core\Form\FormBuilder->doBuildForm('nginx_proxy_settings_form', Array, Object) (Line: 1073)
Drupal\Core\Form\FormBuilder->doBuildForm('nginx_proxy_settings_form', Array, Object) (Line: 1073)
Drupal\Core\Form\FormBuilder->doBuildForm('nginx_proxy_settings_form', Array, Object) (Line: 577)
Drupal\Core\Form\FormBuilder->processForm('nginx_proxy_settings_form', Array, Object) (Line: 323)
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: 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: 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: 707)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

Go to admin/config/system/nginx-proxy-manager-connector/settings

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes