在Chrome扩展程序背景页面中选择文本

Question

我正在开发一个chrome扩展程序，我希望它能够由用户获取所选文本，当我在类似的非扩展页面上运行代码时，代码工作正常，但它在我的后台页面扩展中不起作用。即使我试图在console.log中获取选择的值，它也不会返回任何内容。这是我的javascipt文件：

window.onload = myOnload;
function myOnload(){
  var button = document.getElementById("button");
  chrome.contextMenus.create({
    "title":"MissLang Fix",
    "contexts":["all"],
    "onclick": myClick2 
  });

  document.getElementById("input").innerHTML = "mojtaba";
  button.onclick= function(){myClick()};
}

function myClick2(){
  var txt = '';
  if (chrome.extension.getBackgroundPage().window.getSelection)
    txt = chrome.extension.getBackgroundPage().window.getSelection();
  else if (chrome.extension.getBackgroundPage().document.getSelection)
    txt = chrome.extension.getBackgroundPage().document.getSelection();
  else if (chrome.extension.getBackgroundPage().document.selection)
    txt = chrome.extension.getBackgroundPage().document.selection.createRange().text;
  console.log("this is the console message");
  console.log("txt: " + txt);
}    
function myClick(){
  var myString = document.getElementById("input").value;
  for(var i=0;i<myString.length;i++){
    myString = myString.replace("q","ض");
    myString = myString.replace("w","ص");
  }
  document.getElementById("output").innerHTML = myString;
}

这是我的manifest.json文件：

{
    "name": "MissLang Fix extension",
    "manifest_version": 2,
    "version": "1.0",
    "description": "fixing english to farsi typings.",
    "icons":{
        "16":"icon_16.png",
        "128":"icon_128.png"
    },

    "background":{
        "scripts":["script.js"]
    },

    "browser_action":   {
        "default_icon": "icon.png",
        "default_popup": "missLangFix.html"
    },

    "permissions": [
        "tabs",
        "contextMenus"
    ]

}