Firebase Cloud Function警告（函数返回未定义，预期的承诺或值）

Question

我刚刚设置了一个由sub / pub主题触发的云功能。该函数通过发出GET请求来触发HTTP触发的云函数。一切正常，唯一不完全是警告消息

Function returned undefined, expected Promise or value

我必须承认，我的诺言不如我应有的流利，这可能是导致错误的原因。这是节点代码。

exports.triggerClearHidden = functions.runWith(global.runtimeOptions).pubsub.topic('trig').onPublish((message) => {
        // const pubsubMessage = message.data;
        // console.log(Buffer.from(pubsubMessage, 'base64').toString());
        let urlstr = functions.config().rain.projecturl + "/clearHidden";
        httpGetAsync(urlstr, (txt => console.log(txt)));
});


function httpGetAsync(theUrl, callback)
{
    var XMLHttpRequest = require("xmlhttprequest").XMLHttpRequest;
    var xmlHttp =  new XMLHttpRequest();
    xmlHttp.onreadystatechange = function() { 
        if (xmlHttp.readyState === 4 && xmlHttp.status === 200){
            return callback(xmlHttp.responseText);
        } else {
            return callback("the ready state is " + xmlHttp.readyState + " and the status is " + xmlHttp.status);
        }
    }
    xmlHttp.open("GET", theUrl, true); // true for asynchronous 
    xmlHttp.send(null);
}