Ajax :this.get_element().style is null 为空

A page will work normaly for a while, but eventualy I get the error: 'this.get_element().style' is null or not an object
 
The error occurs in  the function "Sys$UI$_UpdateProgress$_startRequest()"
It fails on : " this.get_element().style.display = 'block';"

 function Sys$UI$_UpdateProgress$_startRequest() {
        if (this._pageRequestManager.get_isInAsyncPostBack()) {
            if (this._dynamicLayout) this.get_element().style.display = 'block';
            else this.get_element().style.visibility = 'visible';
        }
        this._timerCookie = null;
    }

When I removed UpdateProgress - everything is OK

Solution:解决办法

Changing the DisplayAfter property to 50 solved it:

将 UpdateProgress 的显示时间点 设低一些 500 毫秒也行,一般
 
<asp:UpdateProgress runat="server" DisplayAfter="50" ID="UpdatePanel1Progress" AssociatedUpdatePanelID="UpdatePanel1">