SOLUTION TO THIS ISSUE: install the js_cookie module, ensure this module is installed actively in your configuration!
I have a strange issue;
I get the below error on my site after updating multiple modules. Somehow it is looking for a theme called js_cookie. Not sure whats going on.
User warning: The following theme is missing from the file system: js_cookie in Drupal\Core\Extension\ExtensionPathResolver->getPathname() (line 63 of core/lib/Drupal/Core/Extension/ExtensionPathResolver.php).
Drupal\Core\Extension\ExtensionPathResolver->getPathname('theme', 'js_cookie') (Line: 85)
Drupal\Core\Extension\ExtensionPathResolver->getPath('theme', 'js_cookie') (Line: 127)
Drupal\Core\Asset\LibraryDiscoveryParser->buildByExtension('js_cookie') (Line: 87)
Drupal\Core\Asset\LibraryDiscoveryCollector->getLibraryDefinitions('js_cookie') (Line: 66)
Drupal\Core\Asset\LibraryDiscoveryCollector->resolveCacheMiss('js_cookie') (Line: 149)
Drupal\Core\Cache\CacheCollector->get('js_cookie') (Line: 44)
Drupal\Core\Asset\LibraryDiscovery->getLibrariesByExtension('js_cookie') (Line: 58)
Drupal\Core\Asset\LibraryDiscovery->getLibraryByName('js_cookie', 'js-cookie') (Line: 68)
Drupal\Core\Asset\LibraryDependencyResolver->doGetDependencies(Array, Array) (Line: 70)
Drupal\Core\Asset\LibraryDependencyResolver->doGetDependencies(Array) (Line: 41)
Drupal\Core\Asset\LibraryDependencyResolver->getLibrariesWithDependencies(Array) (Line: 87)
Drupal\Core\Asset\LibraryDependencyResolver->getMinimalRepresentativeSubset(Array) (Line: 113)
Drupal\Core\Asset\AssetResolver->getLibrariesToLoad(Object) (Line: 131)
Drupal\Core\Asset\AssetResolver->getCssAssets(Object, , Object) (Line: 321)
Drupal\Core\Render\HtmlResponseAttachmentsProcessor->processAssetLibraries(Object, Array) (Line: 165)
Drupal\Core\Render\HtmlResponseAttachmentsProcessor->processAttachments(Object) (Line: 97)
Drupal\big_pipe\Render\BigPipeResponseAttachmentsProcessor->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(Object, 'kernel.response') (Line: 214)
Symfony\Component\HttpKernel\HttpKernel->filterResponse(Object, Object, 1) (Line: 202)
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: 28)
Drupal\Core\StackMiddleware\ContentLength->handle(Object, 1, 1) (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->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: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 1, 1) (Line: 704)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)
Deprecated function: dirname(): Passing null to parameter #1 ($path) of type string is deprecated in Drupal\Core\Extension\ExtensionPathResolver->getPath() (line 85 of core/lib/Drupal/Core/Extension/ExtensionPathResolver.php).
Drupal\Core\Extension\ExtensionPathResolver->getPath('theme', 'js_cookie') (Line: 127)
Drupal\Core\Asset\LibraryDiscoveryParser->buildByExtension('js_cookie') (Line: 87)
Drupal\Core\Asset\LibraryDiscoveryCollector->getLibraryDefinitions('js_cookie') (Line: 66)
Drupal\Core\Asset\LibraryDiscoveryCollector->resolveCacheMiss('js_cookie') (Line: 149)
Drupal\Core\Cache\CacheCollector->get('js_cookie') (Line: 44)
Drupal\Core\Asset\LibraryDiscovery->getLibrariesByExtension('js_cookie') (Line: 58)
Drupal\Core\Asset\LibraryDiscovery->getLibraryByName('js_cookie', 'js-cookie') (Line: 68)
Drupal\Core\Asset\LibraryDependencyResolver->doGetDependencies(Array, Array) (Line: 70)
Drupal\Core\Asset\LibraryDependencyResolver->doGetDependencies(Array) (Line: 41)
Drupal\Core\Asset\LibraryDependencyResolver->getLibrariesWithDependencies(Array) (Line: 87)
Drupal\Core\Asset\LibraryDependencyResolver->getMinimalRepresentativeSubset(Array) (Line: 113)
Drupal\Core\Asset\AssetResolver->getLibrariesToLoad(Object) (Line: 131)
Drupal\Core\Asset\AssetResolver->getCssAssets(Object, , Object) (Line: 321)
Drupal\Core\Render\HtmlResponseAttachmentsProcessor->processAssetLibraries(Object, Array) (Line: 165)
Drupal\Core\Render\HtmlResponseAttachmentsProcessor->processAttachments(Object) (Line: 97)
Drupal\big_pipe\Render\BigPipeResponseAttachmentsProcessor->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(Object, 'kernel.response') (Line: 214)
Symfony\Component\HttpKernel\HttpKernel->filterResponse(Object, Object, 1) (Line: 202)
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: 28)
Drupal\Core\StackMiddleware\ContentLength->handle(Object, 1, 1) (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->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: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 1, 1) (Line: 704)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)