Hello,

As of revision 899 the following is throwing an exception, I suspect it has something to do with the Theme property on the ext:ScriptManager tag:

Example.aspx:
<%@ Page Language="C#" %>
<%@ Register Assembly="Coolite.Ext.Web" Namespace="Coolite.Ext.Web" TagPrefix="ext" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<script runat="server">
    protected void Page_Load(object sender, EventArgs e)
    {
        Store1.DataSource = new object[] {
            new object[] { 1, "Timothy", "Cool" }
        };
        Store1.DataBind();
    }

    protected void ComboBox1_Select(object sender, EventArgs e)
    {

    }

</script>

<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
    <title>Example Page</title>
</head>
<body>
    <form id="Form1" runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server" EnablePartialRendering="True" />
    <ext:ScriptManager ID="ScriptManager2" runat="server" AjaxViewStateMode="Include" StateProvider="PostBack" Theme="Gray" />
    
    <ext:Store ID="Store1"
        runat="server"
        AutoLoad="True">
        <Reader>
            <ext:ArrayReader ReaderID="CustomerId">
                <Fields>
                    <ext:RecordField Name="CustomerId" />
                    <ext:RecordField Name="Customer" />
                    <ext:RecordField Name="Value" />
                </Fields>
            </ext:ArrayReader>
        </Reader>
    </ext:Store>
    <ext:ComboBox ID="ComboBox1"
        runat="server"
        StoreID="Store1"
        DisplayField="Customer"
        Mode="Local"
        ValueField="Value">
        <AjaxEvents>
            <Select OnEvent="ComboBox1_Select" />
        </AjaxEvents>
    </ext:ComboBox>
    

    </form>
</body>
</html>
Replication steps:

1. Load page
2. Select value from ComboBox1
3. Exception

Resolution:

1. Remove Theme="Gray" from ScriptManager
2. Load page
3. Select value from ComboBox1
4. Works

Cheers,
Timothy