为什么此示例连接在background.js和content.js之间的chrome扩展中失败？

Question

我正在学习Chrome扩展程序不同部分之间传递的消息。如https://developer.chrome.com/apps/messaging中所述，我做了简单的扩展。但是它总是会报错：

Error handling response: TypeError: Cannot read property 'farewell' of undefined
Unchecked runtime.lastError: Could not establish connection. Receiving end does not exist.

我不知道为什么。我只是从chrome网站复制代码。我确实需要帮助解决此问题。 这是我的文件： manifest.json：

{
  "manifest_version": 2,
  "name": "Connect",
  "version": "0.0",

  "background": {
    "persistent": false,
    "scripts": ["background.js"]
  },
  "content_scripts": [{
    "matches": [ "<all_urls>" ],
    "js": ["content.js"]
  }],
"browser_action": {
    "default_title": "Test"
},
"permissions": [
    "activeTab",
    "<all_urls>"
]
}

background.js

chrome.tabs.query({active: true, currentWindow: true}, function(tabs) {
  chrome.tabs.sendMessage(tabs[0].id, {greeting: "hello"}, function(response) {
    console.log(response.farewell);
  });
});

content.js

chrome.runtime.onMessage.addListener(
  function(request, sender, sendResponse) {
    console.log(sender.tab ?
                "from a content script:" + sender.tab.url :
                "from the extension");
    if (request.greeting == "hello")
      sendResponse({farewell: "goodbye"});
  });

如果我做错了什么，请指出。谢谢。