AjaxControlToolkit HTMLEditor returns old content after first time

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ASP.NET

Ajax

custom-controls

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I had requirement that editor should have only Bold, Italic, Underline tools, so I customized Html editor of ajaxcontroltoolkit by inheriting AjaxControlToolkit.HTMLEditor.Editor class. It works fine but It returns old content after first time.
e.g. I browse the page and write - "ABCD" in editor and click first time on Save button. on server I got "ABCD".

I again change the content of editor to "ABCD MNO XYZ" and click on Save button. On server again get "ABCD".

On the save page with other controls of ASP.Net (e.g. TextBox, DropDownList etc.) this is not problem. and I have not used any update panel on the page.

Please help me.