Exception type: NullReferenceException

I have the following scenario: an web part that retrieve an Item from a list

code:

namespace SPPMyWebPart.VWPMyWebPart
{
    [ToolboxItemAttribute(false)]
    public partial class VWPMyWebPart : WebPart
    {
       SPList list = web.Lists["MyList"];

       protected void Page_Load(object sender, EventArgs e)
        {
           int i = Convert.ToInt32(request.QueryString["item"]);
           getItem(i);
        }

       public void getItem(int i)
       {
       SPListItem item = list.GetItemById(id);
       txtTitle.Text = item["Title"];
       }
    }
}

the first time everything happens well

but in a second time i get  this error:

Exception type: NullReferenceException 

message: Object reference not set to an instance of an object ...

in the reference of the "list"