null 为空或不是对象，急求解决！！1

代码如下
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<script type="text/javascript">
var anndelay = 3000;
var anncount = 0;var annheight = 14;var annst = 0;
function announcementScroll() {
if(!annst) {
$('announcementbody').innerHTML += '<br style="clear: both" />' + $('announcementbody').innerHTML;$('announcementbody').scrollTop = 0;
if($('announcementbody').scrollHeight > annheight * 3) {
annst = setTimeout('announcementScroll()', anndelay);
} else {
$('announcement').onmouseover = $('announcement').onmouseout = null;
}
return;
}
if(anncount == annheight) {
if($('announcementbody').scrollHeight - annheight <= $('announcementbody').scrollTop) {
$('announcementbody').scrollTop = $('announcementbody').scrollHeight / 2 - annheight;
}
anncount = 0;annst =

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