What does this error message mean? How can I fix it?
User warning: The following theme is missing from the file system: media in drupal_get_filename() (line 268 of core/includes/bootstrap.inc).
drupal_get_filename('theme', 'media') (Line: 291)
drupal_get_path('theme', 'media') (Line: 394)
Drupal\Core\Asset\LibraryDiscoveryParser->drupalGetPath('theme', 'media') (Line: 85)
Drupal\Core\Asset\LibraryDiscoveryParser->buildByExtension('media') (Line: 87)
Drupal\Core\Asset\LibraryDiscoveryCollector->getLibraryDefinitions('media') (Line: 66)
Drupal\Core\Asset\LibraryDiscoveryCollector->resolveCacheMiss('media') (Line: 149)
Drupal\Core\Cache\CacheCollector->get('media') (Line: 44)
Drupal\Core\Asset\LibraryDiscovery->getLibrariesByExtension('media') (Line: 58)
Drupal\Core\Asset\LibraryDiscovery->getLibraryByName('media', 'form') (Line: 54)
Drupal\Core\Asset\LibraryDependencyResolver->doGetDependencies(Array, Array) (Line: 56)
Drupal\Core\Asset\LibraryDependencyResolver->doGetDependencies(Array) (Line: 31) Drupal\Core\Asset\LibraryDependencyResolver->getLibrariesWithDependencies(Array) (Line: 104)
Drupal\Core\Asset\AssetResolver->getLibrariesToLoad(Object) (Line: 116)
Drupal\Core\Asset\AssetResolver->getCssAssets(Object, ) (Line: 291)
Drupal\Core\Render\HtmlResponseAttachmentsProcessor->processAssetLibraries(Object, Array) (Line: 161)
Drupal\Core\Render\HtmlResponseAttachmentsProcessor->processAttachments(Object) (Line: 45)
Drupal\Core\EventSubscriber\HtmlResponseSubscriber->onRespond(Object, 'kernel.response', Object) call_user_func(Array, Object, 'kernel.response', Object) (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.response', Object) (Line: 191)
Symfony\Component\HttpKernel\HttpKernel->filterResponse(Object, Object, 1) (Line: 173)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 664)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)