Uncaught TypeError: Cannot read property 'uid' of null

I don't know why UID returns null. userSessionCheck () contains code to keep the session after login. user.uid gives the same result And Below is my code and error code

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Uncaught TypeError: Cannot read property 'uid' of null

  $(document).ready(function(){
  auth = firebase.auth();
  db = firebase.database();
  storage = firebase.storage();
  userInfo = auth.currentUser;


  userSessionCheck();
  update_profile();


  function update_profile() {

    var mypage_Ref = db.ref('users/' + userInfo.uid).once('value').then(function(snapshot){
      console.log(snapshot.val());
      profileImageUrl = $('#update_email').val(snapshot.val().profileImageUrl);
      email = $('#update_email').val(snapshot.val().email);
      u_name = $('#update_name').val(snapshot.val().username);
      job = $('#update_job').val(snapshot.val().job);


      var html =
      '<div class="profile">' +
       '<img src="'+profileImageUrl+'" id="profile_img">'+

       '<div class="form_g">'+
         '<label>email</label>'+
        '<input type="text" style="text-align: center" id="update_email" placeholder="'+ email +'">'+
       '</div>'+
       '<div class="form_g">'+
         '<label>name</label>'+
         '<input type="text" style="text-align: center" id="update_name" placeholder="'+u_name+'">'+
       '</div>'+
       '<div class="form_g">'+
         '<label>job</label>'+
         '<h5 id="update_job">'+job+'</h5>'+
       '</div>'+
       '<button type="button" class="s-btn" id="info_submit" style="width: 100%; margin-top: 30px;">update</button>'+
      '</div>';

      $('#profile_form').append(html);


    });
  }
});