jquery对话框在使用autoOpen时有效，但在使用.click时则无效

<script>
$(function() {
    $( "#dialogwindow" ).dialog({
        height: 140,
        width: 400,
        zIndex: 999,
        autoOpen: false,
        modal: true
    });

    $( ".loginbtn" ).click(function() {
        $("#dialogwindow").dialog("open");
        return false;
    });
});
</script>
<script>
$(function() {
    $( "#dialogwindow"