Check Panel Form Validation ?

[Mohammad]

I have a form in an Ext.Net Form.
I wanna prevent submitting form if the form is invalid.
but I get this error form.isValid is not a function in the following javascript line

var valid = form.isValid();

Could you please guide me ?
Thanks.

<script type="text/javascript">
function IsValidForm() {
    var panel = Ext.getCmp('<%# MembershipRegistrationPanel.ClientID %>');
    var form = panel.getForm();
    var valid = form.isValid(); //form.isValid is not a function
    if (!valid) {
        Ext.Msg.show({ icon: Ext.MessageBox.ERROR, msg: 'Wrong Form',
            buttons: Ext.Msg.OK
        });
        return false;
    }
    return true;
}
</script>

<ext:Panel ID="MembershipRegistrationPanel" runat="server" Border="true" Header="true"
    BodyStyle="background: #f2fbff" Title=" Register Form " Icon="Accept">
    <Content>
    .....
    <ext:Button ID="SubmitMember" runat="server" Text=" Submit " Height="20" Width="75"
    Icon="Accept" StyleSpec="margin:0 auto;" ValidationGroup="vg1" CausesValidation="true">
    <Listeners>
        <Click Fn="IsValidForm" />
    </Listeners>
    <DirectEvents>
        <Click OnEvent="SubmitMember_Click">
            <EventMask ShowMask="true"></EventMask>
        </Click>
    </DirectEvents>
</ext:Button>
    </Content>
</ext:Panel>