Error when accessing search results page if index is disabled

Problem/Motivation

If the search index is disabled,

The website encountered an unexpected error. Try again later.

Error: Call to a member function getSearchId() on null in search_api_form_views_exposed_form_alter() (line 603 of modules/contrib/search_api/search_api.module).
Drupal\Core\Extension\ModuleHandler->alter() (Line: 839)
Drupal\Core\Form\FormBuilder->prepareForm() (Line: 285)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 134)
Drupal\views\Plugin\views\exposed_form\ExposedFormPluginBase->renderExposedForm() (Line: 1298)
Drupal\views\ViewExecutable->build() (Line: 393)
Drupal\views\Plugin\views\display\PathPluginBase->execute() (Line: 198)
Drupal\views\Plugin\views\display\Page->execute() (Line: 1690)
Drupal\views\ViewExecutable->executeDisplay() (Line: 81)
Drupal\views\Element\View::preRenderViewElement()
call_user_func_array() (Line: 113)
Drupal\Core\Render\Renderer->doTrustedCallback() (Line: 870)
Drupal\Core\Render\Renderer->doCallback() (Line: 432)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 238)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 231)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare() (Line: 128)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse() (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray()
call_user_func() (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch() (Line: 186)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 53)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle() (Line: 116)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 90)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 53)
Asm89\Stack\Cors->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 741)
Drupal\Core\DrupalKernel->handle() (Line: 19)

Steps to reproduce

1. set up a search view with exposed filters
2. go to /admin/config/search/search-api
3. disable the index
4. go to your search results page

Proposed resolution

Check the status of the index, and if it's disabled, handle it gracefully.

Remaining tasks