AjaxControlToolkit error: javascript runtime error unable to get property 'ui' of undefined or null reference ajax

In this post we will discuss how to resolve AjaxControlToolkit error javascript runtime error unable to get property 'ui' of undefined or null reference ajax.

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Before going to this solution, check this article whether you did the explained things or not.

In my situation, I was using a Ajax Calendar extender control to show date. But the error was coming.

I have registered the AjaxControlToolkit properly in the page like below:

<%@ Register TagPrefix="asp" Namespace="AjaxControlToolkit" Assembly="AjaxControlToolkit" %>

But still the error was coming.

Solution:
I was using a script manager in the master page. I was using the script manager like below:

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

So the solution is instead of using the asp script manager use AjaxControlkit script manager like below:

<%@ Register TagPrefix="ajaxToolkit" Namespace="AjaxControlToolkit" Assembly="AjaxControlToolkit" %>

<ajaxToolkit:ToolkitScriptManager runat="server" ID="sm1"/>

Hope this will work for you!!!