chrome.extension.getBackgroundPage返回null

Question

您好我正在尝试从popup.js执行background.js中的函数，但chrome.extension.getBackgroundPage();始终返回null：

popup.js

  var bkg = chrome.extension.getBackgroundPage();
  bkg.test(function(result) {
    alert(result);
  });

background.js

  function test(func) {
      alert('test');
    func.apply(this, ['bar']);
  }

和我的清单：

{
  "name": "Get pages source",
    "background": {
    "scripts": ["background.js"],
    "persistent": true
  },
  "version": "1.0",
  "manifest_version": 2,
  "description": "Get pages source from a popup",
  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  },
  "permissions": ["tabs", "<all_urls>",
    "downloads","http://*/*", "https://*/*","background"
  ]

}

有人知道为什么吗？