I am having problems with ASP.NET C# code?

[Shorty2]

Can someone please help me. I am getting an error when I run my page and it says

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:


Line 43: string TitleID = ddlTitleID.Text;
Line 44: //avoids duplicate titles
Line 45: if (cart.ContainsKey(TitleID))
Line 46: {
Line 47: }

Here is what my code looks like in my asp.net file:

SortedList cart = this.GetCart();
string TitleID = ddlTitleID.Text;
//avoids duplicate titles
if (cart.ContainsKey(TitleID))
{
}
else
cart.Add(TitleID, item);
}