以下语句错误是什么意思？

document.getElementById("context-addonnotification") is null
[在此错误处中断] document.getElementById("context-add...tion").hidden = gContextMenu.onImage;

（来自firebug）
点击第二行显示一个窗口
以下是内容
addonnotification.onFirefoxLoad = function(event) {
  document.getElementById("contentAreaContextMenu")
          .addEventListener("popupshowing", function (e){ addonnotification.showFirefoxContextMenu(e); }, false);
};

addonnotification.showFirefoxContextMenu = function(event) {
  // show or hide the menuitem based on what the context menu is on
  document.getElementById("context-addonnotification").hidden = gContextMenu.onImage;
};

window.addEventListener("load", addonnotification.onFirefoxLoad, false);