Null reference exception on Edit

Question

I have an Asp.Net application , on Edit click on Gridview a popup is opened , but as some columns may be null i am getting error on assignment.

var teacher= from t in db.Teachers.ToList()
                        select new
                        {
                         Name=t.UserName,
                         FirstName=t.FirstName,
                         LastName=t.LastName,
                         Email=t.Email,
                         Lab=t.Lab
                        }

  txtUserName.Text=teacher.Name;
  txtFirstName.Text=teacher.FirstName;
   txtLastName.Text=teacher.LastName; // here i am getting error
  txtEmail.Text=teacher.Email;
   txtLab.Text=teacher.Lab;