How to read property of 'id' using express-session?

This is my mongoose schema for work:

var mongoose = require('mongoose');

var WorkSchema = mongoose.Schema({
Title:{
    type:String,
    required:true, 
    unique:true
},
   VideoURL:String,
   Image: { data: Buffer, contentType: String },
   Price:Number,
   Username:String,
   Company_id:[{type:mongoose.Schema.Types.ObjectId, ref:'Company'}]

});

var Work = module.exports = mongoose.model("Work", WorkSchema);

And this is the my work controller:

    let Work = require('../Models/Work');
    let Company = require('../Models/Company');
    let WorkController = {
     createWork:function(req, res){

       let   trial = new  Work();
       console.log( req.session.Username+ " session");
       trial.Username= req.session.Username;
       trial.VideoURl= req.body.VideoURl;
       trial.Price=req.body.Price;
       trial.Title= req.body.Title;
       User=req.session.Username;

Company.findOne( {Username:User},function(err, company){
            trial.Company_id=company._id;


            trial.save(function(err, Work){
            if(err){
               res.send(err.message);
               console.log(err);
           }


          });
       });
    }
 }


 module.exports = WorkController;

And this is my schema for Company: let Company = require('../Models/Company');

let CompanyController = {



login:function(req,res){
var Username = req.body.Username;
var  Password =req.body.Password;

 Company.findOne({Username:Username,Password:Password},function(err,Company)
    {
      if(!Company){

              return console.log('you are not athorized ');
 res.status(500).send(errorMsg);

                 ;}
            else{
                 res.json(Company);
                req.session.Username = Company.Username;
                console.log("username session: " + req.session.Username);

              }
        })
  }
,

      createCompany:function(req, res){
         let company = new  Company(req.body);

   company.save(function(err, company){
            if(err){
                res.send(err.message)
               console.log(err);
            }
             else{
  req.session.Username= req.body.Username;

                console.log(company);

            }
         })
    }
}

  module.exports = CompanyController;

When the the company user logs in and tries to create work, this error appears in the console "TypeError: Cannot read property 'id' of null"

via user7632716