Previewing Contents In A Textarea

This works when i call the function with a link... when i close the window, and try and open it again, i get an error.

function previewTextarea() {
previewTextarea = document.getElementById('textarea').value;
if(previewTextarea != "") {
window_previewTextArea_name = "js_popup_win";
window_previewTextArea = window.open("",window_previewTextArea_name,config="width=400,height=400,menubar=0,toolbar=0,scrollbars=1,location=0,directories=0,status=0,resizable=1");
window_previewTextArea.document.write("<a href="javascript:window.close();">[ Close Window ]</a>");
window_previewTextArea.document.write("<hr />");
window_previewTextArea.document.write(previewTextarea);
} else {
alert("The textarea is empty or of a null value. Operation failed.");
}}

any ideas?