Microsoft JScript runtime error: Sys.ArgumentNullException: Value cannot be null. Parameter name: handler

Microsoft JScript runtime error: Sys.ArgumentNullException: Value cannot be null. Parameter name: handler

This is a very common exception in the Microsoft Ajax Library, and just means you've got something named wrong or you are missing a parameter.  Sometimes you might just have the wrong element id set somewhere as a parameter, and this will throw the exception.

So please make sure the necessary property target of some control is still exist on the page after executing the server code. If it is Visible=flase brings on this problem exactly, you can use Style["display"]="none" to hide the control.

So please try to change the code blow:

From:

lnkLeft.Visible =false;

lnkRight.Visible = true;

to:

<style>

 .myclass {

Display:none;

 }

 </style>

lnkLeft.Attributes.Remove("class");

lnkLeft.Attributes.Add("class", " myclass ");