loadHTML无效

Question

我正在尝试构建Chrome扩展程序，以便在Trix中的消息/评论结尾添加个性化签名/消息

从阅读文档和其他类似问题我发现它看起来非常简单。我的content.js文件......

就是这样的

'use strict'
var sig = '<p>Testing 1,2</p>';
//chrome.storage.sync.get(null, function (data) {
//    sig = data.signature;
//});
document.addEventListener("trix-initialize", function (event) {
    event.target.editor.loadHTML(sig);
});

......应该做的伎俩。但我一直收到这个错误...

Uncaught TypeError: Cannot read property 'loadHTML' of undefined at HTMLDocument.<anonymous>

在获得更多指导后，我已将此清单，脚本和内容文件包括在内

的manifest.json

{
    "manifest_version": 2,
    "name": "BaseCamp Signature", 
    "description": "This extension adds signature to Basecamp", 
    "version": "1.0",
    "browser_action": {
        "default_icon": "icon.png",
        "default_popup": "popup.html",
        "default_title": "Title!"
    },
    "permissions": [
        "storage", 
        "activeTab",
        "tabs",
        "https://3.basecamp.com/*"    
    ],
    "content_scripts": [
        {
            "matches": ["https://3.basecamp.com/*/*"], 
            "all_frames": true,
            "js": ["content.js"],
            "run_at": "document_end"
        }
    ],
    "web_accessible_resources": [
        "script.js",  
    ]
}

的script.js

'use strict'
var sig = '<p>Testing 1,2</p>';
//chrome.storage.sync.get(null, function (data) {
//    sig = data.signature;
//});
document.addEventListener("trix-initialize", function (event) {
    event.target.editor.loadHTML(sig);
});

content.js

var s = document.createElement('script');
// TODO: add "script.js" to web_accessible_resources in manifest.json
s.src = chrome.extension.getURL('script.js');
s.onload = function() {
    this.remove();
};
(document.head || document.documentElement).appendChild(s);