Unable to access panel from script

In my browser.xul file I have a panel element:

<panel id="panel1">
   <!-- Elements -->
</panel>

When I try:

window.document.getElementById("panel1").openPopup(button, "after_start", 0, 0, false, false);

the browser says that:

window.document.getEle开发者_Python百科mentById("panel1") is null.

Am I missing anything here?

it's my first time I see panel tag in html..

are you sure you don't mean :

<div id="panel1">
....
</div>