Tab key problem in Modal pop up extebder

function pageLoad(sender, args) {
    $find('ModalPopupBehavior').add_shown(Shown);
    $find('ModalPopupBehavior').add_showing(Showing);
    $find('ModalPopupBehavior').add_hidden(Hidden);
}

function Shown(sender, args) {
    modalId = sender.get_id();

}
function Showing(sender, args) {
    if (modalId == 'ModalPopupBehavior') {
        args.set_cancel(true);
    }
}
function Hidden(sender, args) {
    modalId = '';
}