I have the following class, of which I want to store a list inside a ViewState.

[Serializable]
public class Project
{
    public string projectID { get; set; }
    public string workerID { get; set; }
    public string nameProject{ get; set; }

    public Project(string pid, string wid, string npro)
   {
      projectID = pid;
      workerID = wid;
      nameProject= npro;
  }
}

Within my aspx I have the following method that fills a list with the projects that I get from a DataTable. I finally store the list in a ViewState for later use.

private void fillList(DataTable dt)
{
    List<Project> projects = new List<Project>();

    for (int i = 0; i < dt.Rows.Count; i++)
    {
        string pid = dt.Rows[i][0].ToString();
        string wid = dt.Rows[i][1].ToString();
        string npro = dt.Rows[i][2].ToString();

        projects.Add(new Project(pid, wid, npro));
    }

    ViewState["Projects"] = new List<Project>();
    ViewState["Projects"] = projects;
}

Now I want to retrieve the list stored in the ViewState, to perform other actions. This is what I have tried.

protected void btnRetreive_Click(object sender, EventArgs e)
{                
    List<Project> data = (List<Project>)ViewState["Projects"];

    Response.Write(data.Count);   
}

But when I run it I get the following error

An exception of type 'System.NullReferenceException' occurred in App_Web_vwe5qgkl.dll but was not handled in user code

Additional information: Object reference not set to an instance of an object.

What am I doing wrong?