鼠标移到gridview上显示全部内容成功了，但是原来的编辑功能失效了？

RT
我使gridview每个行上显示字符数固定，剩下的显示省略号，同时鼠标移动到上面显示全部内容，成功了，但是原来的编辑功能，却出现了未实例化的问题！求解啊！                      <asp:GridView ID="GridView1" 
                        runat="server" Height="373px" Width="628px" 
                        AutoGenerateColumns="False" CellPadding="2" ForeColor="#333333" 
                        GridLines="None" 
                        AllowPaging="True" PageSize="8" 
                        onpageindexchanging="GridView1_PageIndexChanging" 
                        style="margin-left: 0px" DataKeyNames="homeworkid,teacherid,lessionid,subject,optiona,optionb,optionc,optiond,answer" 
                        onrowcancelingedit="GridView1_RowCancelingEdit" 
                        onrowdeleting="GridView1_RowDeleting" onrowediting="GridView1_RowEditing" 
                        onrowupdating="GridView1_RowUpdating" onrowdatabound="GridView1_RowDataBound">
                        <RowStyle BackColor="#E3EAEB" />
                        <Columns>
                            <asp:TemplateField HeaderText="题目">
                                <EditItemTemplate>
                                    <asp:TextBox ID="TextBox6" runat="server" Text='<%#Eval("subject")%>'></asp:TextBox>
                                </EditItemTemplate>
                                <ItemTemplate><asp:Label 
                                        ID="L11" runat="server" Text='<%# Eval("subject").ToString().Length > 5 ? Eval("subject").ToString().Substring(0, 5)+ "..." : Eval("subject") %>'></asp:Label>
                                       <asp:Label ID="L1" runat="server" Text='<%#Eval("subject")%>' Visible="False"></asp:Label>
                                </ItemTemplate> </asp:TemplateField>
<asp:CommandField ShowDeleteButton="True" />
                        </Columns>                        <FooterStyle BackColor="#1C5E55" Font-Bold="True" ForeColor="White" />
                        <PagerStyle BackColor="#666666" ForeColor="White" HorizontalAlign="Center" />
                        <SelectedRowStyle BackColor="#C5BBAF" Font-Bold="True" ForeColor="#333333" />
                        <HeaderStyle BackColor="#1C5E55" Font-Bold="True" ForeColor="White" />
                        <EditRowStyle BackColor="#7C6F57" />
                        <AlternatingRowStyle BackColor="White" />
                    </asp:GridView>cs代码：    protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            ((Label)e.Row.Cells[0].FindControl("L11")).ToolTip = ((Label)e.Row.Cells[0].FindControl("L1")).Text;                    }//一旦点击编辑，报错了，说它为实例化    }