当搜索与文档不匹配时，Mongoose Library find / findOne Method不返回值

我正面临着与Mongoose有关的一个问题 . 当我使用find或findOne方法并且没有匹配结果时，回调函数不返回null / err并挂起进程 . 使用MongossDB V3.4的Mongoss 5.1.5 . 请指教

module.exports.validateappsubscripition = function (userid, appkey, cb) {
 //console.error(userid + ' ' + appkey)
 var userobj_id = mongoose.Types.ObjectId(userid);
 appsubscripitions.model.findOne({'subscribersuserid': userobj_id , 'appkey' 
  :appkey }, function(err,doc){
 console.error('test2');
 if(doc ){
 cb(null, doc );
 }
 else{ 
    cb(null, null );
 }

} ); 
}

调用阻止：尝试从req标头验证密钥 . 我试图从下面的块调用函数validateappsubscripition .

module.exports.sendEmail = function (req, res, next) {
let appkey; 
let userid;
if (req.headers.appkey) {
    appkey = req.headers.appkey;
    console.error(appkey);
}
else {
    appkey = '';
}

if(req.user._id){
    userid = req.user._id ; 
    console.error(userid);
}

if (!req.body.fromEmail || !req.body.toEmail || !req.body.subject || !req.body.content) {
    res.json({ success: false, msg: 'Please pass all the required parameters' });
    next();
}


appsubcripitions.validateappsubscripition(userid, appkey, function (err, doc) {
    console.error('test2');
    if (err) {
        res.json({ success: false, msg: 'Unauthorized. App Key is misssing on the header or App key is not valid' });
        next();
    }
    else if (doc ) {

        this.getSMTP('smtp.gmail.com', 'arbalu@gmail.com', function (err, userInfo) {
            if (err) {
                res.json({ success: false, msg: err.message });
                next();
            }
            if (userInfo) {
                //userInfo = user;
                var send = require('gmail-send')({
                    user: userInfo.user,
                    pass: userInfo.pass,
                    from: req.body.fromEmail,
                    to: req.body.toEmail,
                    subject: req.body.subject,       // Override value set as default
                    text: req.body.content
                });


                send({ // Overriding default parameters 
                    // to: req.toEmail,
                    // subject: req.subject,       // Override value set as default
                    // text: req.content
                    // files: [filepath],
                }, function (err, response) {
                    //console.log('* [example 1.1] send() callback returned: err:', err, '; res:', res);
                    if (err) {
                        res.json({ success: false, msg: err.message });
                        next();
                    }
                    else {
                        res.json({ success: true, msg: response });
                        next();
                    }
                });

            }
        })
    }
    else {
        res.json({ success: false, msg: 'Some issue on sending email.Please contact the support.' });
        next();
    }
});

}