how slove this error:UpdateParameters

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ASP.NET

how to slove this problam

if (e.CommandName == "Update")
      {
          GridViewRow gvrow = (GridViewRow)((ImageButton)e.CommandSource).NamingContainer;
          TextBox txtbarndFaName = (TextBox)gvrow.FindControl("TFBrandFaNameedit");
          TextBox txtbarndEnName = (TextBox)gvrow.FindControl("TFBrandEnNameedit");
          DropDownList txttypepartEnName = (DropDownList)gvrow.FindControl("DDTypehardware");
          int a = int.Parse(txttypepartEnName.SelectedValue);
          SqlDupdate.UpdateParameters["FBrandFaName"].DefaultValue = txtbarndFaName.Text;
          SqlDupdate.UpdateParameters["FBrandEnName"].DefaultValue = txtbarndEnName.Text;
       SqlDupdate.UpdateParameters["FPartCatID"].DefaultValue = txttypepartEnName.SelectedValue.ToString();

          SqlDupdate.Update();
      }

this error:
System.NullReferenceException: Object reference not set to an instance of an object.

DropDownList txttypepartEnName = (DropDownList)row.FindControl("DDTypehardware

the type filed db is smallint.
how to convert SqlDupdate.UpdateParameters["FPartCatID"].DefaultValue = txttypepartEnName.SelectedValue.ToString();

to int.