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

hi guys  transferring  selected rows of gridview to another gridview with button click. and vice versa by selecting checkbox  No data is present in second gridview. . but i am getting above error:here is my code:
protected void cmdRight_Click(object sender, EventArgs e) { DataTable dtLeft = (DataTable)ViewState["dtLeft"]; DataTable dtRight = (DataTable)ViewState["dtRight"]; int tmp = 0; foreach (GridViewRow oItemLeft in GridView1.Rows) { if (((CheckBox)oItemLeft.FindControl("chk")).Checked) { dtRight.Rows.Add(GridView1.DataKeys[oItemLeft.DataItemIndex].Value, oItemLeft.Cells[1].Text, oItemLeft.Cells[2].Text); dtLeft.Rows.RemoveAt(oItemLeft.DataItemIndex - tmp); tmp++; } } GridView2.DataSource = dtRight; GridView2.DataBind(); ...