HtmlEditorExtender loses its text during postback

Hi,
In my project , When I type texts in the HtmlEditorExtender, and click on a button. Everything works fine. But when I copy & Paste texts (short , a word ,paragraphs etc). The HtmlEditorExtender loses its data. 

my code looks like this: Default.aspx

<asp:UpdatePanel runat="server" ID="id123"><Triggers><asp:PostBackTrigger ControlID="Button1" /></Triggers><ContentTemplate><asp:TextBox ID="txtSlidePage"  TextMode="MultiLine" Columns="50" Rows="10" Width="100%" Height="200" runat="server" /><ajaxToolkit:HtmlEditorExtender ID="HtmlEditorExtender1" TargetControlID="txtSlidePage"   runat="server" DisplaySourceTab="true"><Toolbar><ajaxToolkit:Undo /><ajaxToolkit:Redo /></Toolbar></ajaxToolkit:HtmlEditorExtender></ContentTemplate></asp:UpdatePanel><asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click1"  />

Default.aspx.cs:

 protected void Button1_Click1(object sender, EventArgs e)
    {

        Response.Write("<script>alert('" + txtSlidePage.Text + "');</script>");
    }

Any Ideas about this issue?