Sys.WebForms.PageRequestmanagerServerErrorException

Sys.Webforms.PageRequestManagerServerErrorException: Object reference not set to an instance of an object.

I was getting this error when I clicked on an object that's inside the UpdatePanel.

The solution was to ..

EnablePartialRendering="true" to "false"

<asp:ScriptManager ID="ScriptManager1" runat="server" ScriptMode="Release" EnablePartialRendering="true">

to 

<asp:ScriptManager ID="ScriptManager1" runat="server" 

ScriptMode="Release"EnablePartialRendering="false">