无法从Gridview中获取值

我想通过我的gridview做一个更新操作，但我不能弄清楚。

我的gridview是这样的

<asp:GridView ID="GridView1" runat="server" ShowFooter="True" AutoGenerateColumns="False" DataKeyNames="ID"
                OnDataBinding="GridView1_DataBinding"
                OnRowDeleting="GridView1_RowDeleting"
                OnRowEditing="GridView1_RowEditing"
                OnRowUpdating="GridView1_RowUpdating">
                <Columns>
                    <asp:CommandField ShowDeleteButton="True" ShowEditButton="True" />
                    <asp:BoundField DataField="ID" HeaderText="ID" ReadOnly="True" SortExpression="UserID" />
                    <asp:BoundField DataField="UserName" HeaderText="UserName" SortExpression="UserName" />
                    <asp:BoundField DataField="UserPW" HeaderText="UserPW" SortExpression="UserPW" />
                    <asp:BoundField DataField="UserType" HeaderText="UserType" SortExpression="UserType" />
                </Columns>
            </asp:GridView>

我的添加/删除工作得很好，但有一个问题与更新，如从网格获取空值。我尝试了3种不同的方法来获得值。注释行给了我NullReferenceException，而未注释的行没有，但是它们保存的值是空字符串。

这是我的aspx.cs更新。(我使用GridView的编辑/更新)

protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)
        {
            GridView1.EditIndex = e.NewEditIndex;
            GridView1.DataBind();
        }
        protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)
        {
            int userID = Convert.ToInt32(this.GridView1.DataKeys[e.RowIndex]["ID"]);
            var UserOperations = new ModelOperations.UserList.UserListOperations();
            //TextBox uname = (TextBox)this.GridView1.Rows[e.RowIndex].FindControl("UserName");
            string uname = this.GridView1.Rows[e.RowIndex].Cells[2].Text;
            string upw = this.GridView1.Rows[e.RowIndex].Cells[3].Text;
            //string uname = this.GridView1.SelectedRow.Cells[2].Text;
            //string upw = this.GridView1.SelectedRow.Cells[3].Text;
            //TextBox upw = (TextBox)this.GridView1.Rows[e.RowIndex].FindControl("UserPW");
            var result = UserOperations.updateUser(userID, uname, upw);
            GridView1.EditIndex = -1;
            GridView1.DataBind();
        }

string uname = e.NewValues["UserName"];
string upw = e.NewValues["UserPW"];