Rows cannot be programmatically added to the DataGridView's rows collection when the control is data-bound.

Hi, i use below code to add new row to my datagridview which has a dataset as datasource.it gives an error saying "Rows cannot be programmatically added to the DataGridView's rows collection when the control is data-bound" when i use

datagridview1.Rows.Add()

. So i searched on internet found below solution.But now it gives an NullReferenceException saying "Object reference not set to an instance of an object."
 

// ds_ITEMS is my dataset name.

DataRow dr = ds_ITEMS.Tables["StkTrx"].NewRow(); // NullReferenceException gives in this line
            dr["Code"] = this.txtCode.Text;
            dr["ItemDes"] = this.txtDes.Text;
            dr["Qty"] = this.txtQty.Text;
            dr["UnitPrice"] = this.txtCostPrice.Text;
            dr["Amount"] = this.txtAmount.Text;
 
            ds_ITEMS.Tables["StkTrx"].Rows.Add(dr);

 
Any suggestions?