AJAX Exception "Object reference not set to an instance of an object."

[Tamer Ibrahim]

Hi,
Sometimes, I got the following error message when I use ajax calendar
control on some aspx pages :
Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the
current web request. Please review the stack trace for more information
about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set
to an instance of an object.

Source Error:

An unhandled exception was generated during the execution of the
current web request. Information regarding the origin and location of the
exception can be identified using the exception stack trace below.

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an
object.]
AjaxControlToolkit.ScriptObjectBuilder.RegisterCss References(Control
control) in
d:\E\AjaxControlToolkit\Release\AjaxControlToolkit \ExtenderBase\ScriptObjectBuilder.cs:229
AjaxControlToolkit.ExtenderControlBase.OnPreRender (EventArgs e) in
d:\E\AjaxControlToolkit\Release\AjaxControlToolkit \ExtenderBase\ExtenderControlBase.cs:398
System.Web.UI.Control.PreRenderRecursiveInternal() +77
System.Web.UI.Control.PreRenderRecursiveInternal() +161
System.Web.UI.Control.PreRenderRecursiveInternal() +161
System.Web.UI.Control.PreRenderRecursiveInternal() +161
System.Web.UI.Control.PreRenderRecursiveInternal() +161
System.Web.UI.Control.PreRenderRecursiveInternal() +161
System.Web.UI.Page.ProcessRequestMain(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1360