将值分配给列表元素不起作用

Question

以下代码中是否缺少某些内容，这会引发错误：
 对象引用未设置为对象的实例。

//In bllProgress.cs file
namespace BussinessLogic
{
    public class bllProgress
    {
        public List<Timeline> BTimeline { get; set; }
    }
    public class Timeline
    {
        public string DateTime { get; set; }
        public string Heading { get; set; }
    }
}

//In Controller.cs file
using BussinessLogic;
public PartialViewResult FireTimeline()
{
    Timeline tl = new Timeline();
    tl.DateTime = "Sign Date";
    tl.Heading = "Aggrement";

    bllProgress obj = new bllProgress();
    obj.BTimeline.Add(tl); // <--------- This line gives an error.
    return PartialView("_Timeline", obj);
}