Add items to Generic List Collection in C#

How to Add items to Generic List Error

i am getting error "Object reference not set to an instance of an object."

    public  class Employee {
        public string Name { get; set; }
        public List<EmployeeDepartments> EmployeeDetails { get; set; }
    }


    public class EmployeeDepartments {
        public string Department { get; set; }
        public int Mark { get; set; }
    }


    private void Form1_Load(object sender, EventArgs e) {
            List<Employee> employees = new List<Employee>();
            try 
            {
                Employee emp = new Employee();
                emp.Name = "a";
                EmployeeDepartments dept = new EmployeeDepartments();
                dept.Department = "IT";
                dept.Mark = 100;
                emp.EmployeeDetails.Add(dept);
                employees.Add(emp);
            } 
            catch(Exception ex) {                
            }
        }