RadTextBox script error: Script control 'myRadTextBox' is not a registered script control

[Edith]

I got this error when I create a Radtextbox control in my page.

Script control 'txtRechazoComentarios' is not a registered script control. Script controls must be registered using RegisterScriptControl() before calling RegisterScriptDescriptors().
Parameter name: scriptControl

My aspx goes like:

<telerik:RadMultiPage ID="rmpTabs" runat="server" SelectedIndex="0">
    <telerik:RadPageView ID="rpvSolicitud" runat="server">
        ...
        <telerik:RadAjaxPanel ID="rapAutorizaRechaza" runat="server">
            <asp:Panel ID="PanelAutorizaRechaza" runat="server" Visible="false">
                ...
                <telerik:RadTextBox ID="txtRechazoComentarios" runat="server" Visible="true" TextMode="MultiLine" Width="98%" Height="98%" >
                    <ClientEvents OnLoad="MostrarTxtRechazo" />
                </telerik:RadTextBox>
                ...
            </asp:Panel>
        </telerik:RadAjaxPanel>
        ...
    </telerik:RadPageView>
</telerik:RadMultiPage>

and the scripts:

function MostrarTxtRechazo() {
            var rechazo = document.getElementById('hdnRechazar');

            if (rechazo != null && rechazo.value == 1 && (TieneInnerHTML('<%= lblErrorMotivo.ClientID %>') || TieneInnerHTML('<%= lblErrorPassword.ClientID %>'))) {
                MostrarElemento('lblRechazoComentarios');
                MostrarElemento('divRechazoComentarios');
            }
        }

I already tried the RegisterWithScriptManager="false" property, but hen I got some weird error saying:
Microsoft JScript runtime error: Object doesn't support property or method '$'
at Telerik.Web.UI.WebResource.axd
var d=$telerik.$(a);

can you guide me through this?
thanks

-E

[Edith]

I  think I found the solution, just removed the "Visible=false" from the outside asp:Panel and everything worked as a charm

Now I have to figure out how to hide/show this panel... anyway... thanks!