Error: "Unable to get property 'UI' of undefined or null reference" when using ModalPopupExtender

I was using below code combination for opening model popup control, while run time I got this error.

asp:ScriptManager
asp:ModalPopupExtender

To resolve this error I just replaced "asp:ScriptManager" tag with "asp:ToolkitScriptManager" and it's works for me.

asp:ToolkitScriptManager
asp:ModalPopupExtender