使用indexedDB时sendMessage没有响应

Question

我正在使用indexedDB编写chrome扩展，并希望使用sendMessage将数据从后台页面传递到弹出窗口。
如果按下按钮，则会从弹出窗口向背景发送消息，并且应将响应写入“结果”div。 日志消息都没问题，但是没有从'openRequest.onsuccess'块中发送响应。

我不确定为什么没有从openRequest.onsuccess收到响应，但是在写入之后就是控制台消息。

来自console.log的

输出;
弹出;
processMessage（） - 2

背景;
processMessage（） - 2
processMessage（） - 1

'结果'的内容;
processMessage（） - 2

popup.js

var popupInst;
var popupPage = function() {
    var mThis = this;
    this.msg;
    this.init = function() {
        document.querySelector('#btnAdd').addEventListener('click', this.add);
    };
    this.add = function() {
        chrome.extension.sendMessage({ add: { keyName: 'key', valueName: 'value' }}, function(response) {
            mThis.msg = (response.msg)
            console.log(mThis.msg);
            $('#results').empty().append(mThis.msg);
        });
    };
}
document.addEventListener('DOMContentLoaded', function(){popupInst = new popupPage(); popupInst.init();});

background.js

var bgInst;
var backgroundPage = function()
{
    var mThis = this;
    this.dbName = 'test-db';
    this.db;
    this.init = function() {
        var openRequest = window.indexedDB.open(mThis.dbName,1);
        openRequest.onsuccess = function(evt) {
            this.db = openRequest.result;
        };
        openRequest.onupgradeneeded = function(evt) {
            var objStore = evt.target.result.createObjectStore(this.dbName,{keyPath:"keyName"});
            objStore.createIndex("keyName","keyName",{unique:true});
            objStore.createIndex("valueName","valueName",{unique:false});
        };
        chrome.extension.onMessage.addListener(this.processMessage);
    };
    this.processMessage = function(msgRequest,msgSender,msgCallback) {
        if(msgRequest.add) {
            var openRequest = window.indexedDB.open(mThis.dbName);
            openRequest.onsuccess = function(evt) {
                var str1 = 'processMessage() - 1';
                msgCallback({ msg: str1 }); -> this message is never received in popup.js
                console.log(str1);          -> but this message is written to the log
            };
            var str2 = 'processMessage() - 2';
            msgCallback({ msg: str2 });
            console.log(str2);
        }
    };
}
document.addEventListener('DOMContentLoaded', function(){bgInst = new backgroundPage(); bgInst.init();});