title: Cannot call method 'create' of undefined chrome-extension
body: <p>when i finish my chrome extension i find an error it is:</p>
  
  <pre><code> Uncaught TypeError: Cannot call method 'create' of undefined chrome-extension://dpfgkakomehcgdcnigflmkcfngeaamlc/js/background.js:46
  </code></pre>
  
  <p>injectScript startLiveReload init  </p>
  
  <p>i use it is this</p>
  
  <blockquote>
    <p>mainfest.json "background": {
        "scripts": ["js/background.js"]   }</p>
  </blockquote>
  
  <p>and background.js like</p>
  
  <pre><code>var readMenuItemId = chrome.contextMenus.create({
    "title" : "阅读模式",
    "type" : "normal",
    contexts : [ "selection" ],
    documentUrlPatterns: ["http://*/*", "https://*/*"],
    "onclick" : genericOnClick
  });
  </code></pre>
  
  <p>i do not know where is wrong</p>