DropdownList with Modal

Good afternoon. I have a screen with a grid made in webforms, and clicking on the button in the grid, a modal with the corresponding registry information is displayed. In this modal I have some dropdownlist attached to tables. Only if I put the OnSelectedIndexChanged event does not open the modal anymore. Has anyone ever had this problem? Modal call:

System.Text.StringBuilder sb = new System.Text.StringBuilder();
            sb.Append(@"<script type='text/javascript'>");
            sb.Append("$('#editModal').modal('show');");
            sb.Append(@"</script>");
            ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "EditModalScript", sb.ToString(), false);

Here is the dropdownlist creation code:

<asp:DropDownList ID="DDLEditReq" runat="server" DataSourceID="SQLEditRec" style="width:340px"
                                            DataTextField="nome_usuario" DataValueField="id_usuario" ValidationGroup="DDL" AutoPostBack="True"
                                            OnSelectedIndexChanged="ddlb_req"></asp:DropDownList>

And here is the code for creating the modal:

 <div id="editModal"  tabindex="-1" class="modal hide fade" role="dialog" aria-labelledby="editModalLabel" aria-hidden="true">