How do I fix this error? I am getting an error right after creating the tweet.libraries.yml file as per istructions given from a book about Drupal 8 theming. Can anyone help me fix this error? 


  The website encountered an unexpected error. Please try again later.
  Error: Cannot use assign-op operators with overloaded objects nor string offsets in Drupal\Core\Asset\LibraryDiscoveryParser->buildByExtension() (line 138 of core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php).
  Drupal\Core\Asset\LibraryDiscoveryParser->buildByExtension('tweet') (Line: 87)
  Drupal\Core\Asset\LibraryDiscoveryCollector->getLibraryDefinitions('tweet') (Line: 66)
  Drupal\Core\Asset\LibraryDiscoveryCollector->resolveCacheMiss('tweet') (Line: 148)
  Drupal\Core\Cache\CacheCollector->get('tweet') (Line: 44)
  Drupal\Core\Asset\LibraryDiscovery->getLibrariesByExtension('tweet') (Line: 58)
  Drupal\Core\Asset\LibraryDiscovery->getLibraryByName('tweet', 'bootstrap') (Line: 54)
  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) (Line: 111)
  Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.response', Object) (Line: 179)
  Symfony\Component\HttpKernel\HttpKernel->filterResponse(Object, Object, 1) (Line: 161)
  Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 62)
  Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
  Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
  Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 98)
  Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 77)
  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: 628)
  Drupal\Core\DrupalKernel->handle(Object) (Line: 19)


This is the content of the tweet.info.yml file.

    name: tweet
    type: theme
    description: 'A twitter Bootstrap starter theme'
    core: 8.x
    base theme: false

    libraries:
      - tweet/bootstrap


This is the content of the tweet.libraries.yml file.

    bootstrap:
      version: 3.3.6
      css:
        theme:
          css/bootstrap.min.css: {}
          css/bootstrap-theme.min.css: {}
      js:
        js/bootstrap.min.js
      dependencies:
        - core/jquery
Change the tweet.info.yml file as follows.

name: 'tweet'
description: 'A twitter Bootstrap starter theme'
core: 8.x
type: theme
base theme: false

libraries:
  - 'tweet/bootstrap'


Put the following in the tweet.libraries.yml file.

bootstrap:
  version: 3.3.6
  css:
    theme:
      css/bootstrap.min.css: {}
      css/bootstrap-theme.min.css: {}
  js:
    js/bootstrap.min.js: {}
  dependencies:
    - core/jquery


Also, be sure you have a tweet.theme file in the same directory. This file is necessary for Drupal to recognize the directory contains a theme.