为什么defaultTableModel会经历nullPointerException？

Question

为什么我的defaultTableModel会经历nullPointerException？

<script>
    document.addEventListener('keydown', function(e){
        //37 is left arrow, 39 is right arrow
        if(e.which === 37){
            document.getElementById('previousPhoto').getElementsByTagName('a')[0].click();
        } else if (e.which === 39) {
            document.getElementById('nextPhoto').getElementsByTagName('a')[0].click();

    }
});

</script>